Boyfriend Bluff

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When a man (and it's almost always a man) saves a girl on the street from being harrassed by some undesirables by coming over and pretending to be her boyfriend or brother, or something similar. The girl is usually not aware that the guy is about to do this, and often doesn't even know the guy in question. This doesn't stop them from turning into real lovers, though. The idea behind this trope is that a man can only be dissuaded from going after a woman if he knows she's already taken, regardless of her level of interest in him.

Compare The Beard, Fake-Out Make-Out.

Examples of the Boyfriend Bluff include:

Advertising

In a commercial for Dentyne Ice gum, a man at a laundromat is throwing increasingly moronic pickup lines at a woman who is showing increasing irritation. Finally, another guy walks up, says, "Hi, honey," and kisses her, then gives her a pack of said gum she'd "wanted". The guy trying to pick her up quickly excuses himself...and when he's gone, the guy who kissed her introduces himself, shaking her hand.

Anime and Manga

In the film version of Howl's Moving Castle, Howl gets Sophie away from some soldiers who are trying to pick her up by coming over and pretending to be her boyfriend.
Subverted in A Certain Scientific Railgun, where Touma attempts to do this with Mikoto. The jig is up before it even begins.
Used in an odd sense in Ai Yori Aoshi. Kaoru dissuades a pair of guys from making a move on Aoi with this claim... the kicker being he really is her fiance, and he'd just kicked her out of his home suspecting her of being a ploy by his estranged family to get him to return (only to immediately realize his mistake). The incident kicks off the two's relationship.
Inverted in the hentai anime Sex Friend. Mina brushes off a persistent ex-lover by grabbing protagonist Takabe and claiming him as her boyfriend.
Fruits Basket: Kyo pulls a non-verbal version of this to protect Tohru from some guys that want to take advantage of her.
In Clannad, in a rare reversal of roles, Nagisa pretends to be Tomoya's girlfriend at one point to get Sunohara to stop bugging Tomoya. She thought that Sunohara was gay and in love with Tomoya... just another one of Tomoya's pranks (in reality, Sunohara just wanted to get Tomoya to play basketball with him for totally unrelated reasons). Note that this only applies to the anime adaptation... in the Visual Novel's equivalent scene, Nagisa actually is Tomoya's girlfriend at that point.
Attempted by one character in Blue Drop, but it just makes the Arume he's bluffing threaten to impregnate the woman in question.
In Seitokai Yakuindomo, Aria escapes from two persistent pickup-artists at the beach by claiming that Takatoshi was her husband. Suzu is less than pleased at being treated as their child.

Comic Books

Bobby and Cecilia do this to throw off the Sentinels in the X-Men comics in the 90s.

Fan Works

Chasing Amy by "echo", a Buffy the Vampire Slayer/Harry Potter story, really kicks off when Marcus Flint claims to be the fiance of Amy (Madison) Malfoy, in order to chase off Parker Parkinson, elder brother of Pansy.

Film

Memorably done by John Cusack in Say Anything.
Nick and Norah's Infinite Playlist, in which the girl actually request it: "Be my boyfriend for five minutes."
Done in Disney's Aladdin, though in this case Al pretends Jasmine is his Cloudcuckoolander sister. Points to Jasmine for immediately getting it and playing along.

Aladdin: Come on sis, time to go see the doctor.
Jasmine: * to camel* Oh hello doctor, how are you?
Aladdin: Uh uh, not that one...

Tank Girl. While Tank Girl and Jet Girl are in Water and Power's prison, Sergeant Small hits on Jet Girl. Tank Girl happens to be nearby and...

Tank Girl: Quit pickin' on my girlfriend! [makes out with Jet Girl]
Sergeant Small: God, that is... [disgusted look, leaves]
Jet Girl: Thanks. It worked.
Tank Girl: [mischievous look] What worked?
Jet Girl: [disgusted look, leaves]

Played with in Predator 2. Jerry Lambert (Bill Paxton) is at a bar. He goes over to a man talking to a woman.

Jerry: Hey, do you mind? It's my sister. [man leaves] Oh, you're not my sister.

Mr. and Mrs. Smith: it's how they met. They actually managed to avoid the military this way: they were looking for an American traveling alone.
Black Widow (1987). Alexandra Barnes and Catharine Petersen are taking a scuba diving class. An unattractive woman with a cold asks to be Catherine's partner, but Alexandra intervenes and says that she and Catharine had already agreed to be partners, much to Catharine's relief. This example fits the trope because there was considerable subsequent Les Yay between Alexandra and Catharine, including two kisses.
Gender Flipped in Legally Blonde, where Elle pretends a boy at her college broke her heart, in order to make him seem cooler and get the Girl Posse to stop haranguing him.
In Little Children , Brad and Sarah (who are married, but not to each other) hug and kiss once to freak out the yuppies at the playground. It works.
Hitch pretended to be the boyfriend of a woman at a bar to get rid of a guy she obviously was not interested in.
In Under the Tuscan Sun, Frances grabs Marcello and pretends to be his girlfriend to escape from the men following her. This is an interesting case in that unlike many other such stories, this one doesn't work out in the end.
In Film Geek, Niko simultaneously wards off and puts down Brandon by telling him Scotty, whom Brandon called "a spaz," is her boyfriend, and pulls him in for a kiss for good measure. Brandon is not fooled, but leaves her alone.
Mentioned in Turistas: "I'm here with my little sister. I get to pretend to be the boyfriend when creepy guys try to bother her."

Literature

Anita Blake takes hitman Bernardo with her to a werewolf biker-gang bar, because she believes they are unlikely to listen seriously to a woman who shows up at their hangout alone.
Considering the above example, Granny needing to take Greebo with her to the opera in Maskerade may count.
In John Green's Paper Towns, Q goes out pranking with Margo at the beginning of the book. Someone hits on her and makes a comment about her "little brother," ie, Q. She tells the clerk, "Actually, he's my cousin." * grabs ass* "And my lover."
Happens all the frickin' time in Genesis, chapter 20 being a good example. It never ends well, but they keep doing it.
- True, Abraham convinces Sarah to pose as his sister, but the reason was that he himself didn't want to be harmed because of men's attraction to her.
Hoyt does this for Charlotte in I am Charlotte Simmons, pretending to be her brother to protect from a drunk hockey player. He does this largely because he wants to be romantically involved with her. Since his interest in her is very nearly as predatory....
In the book, Son of the Mob, this is how the titular mob boss's son meets and eventually starts dating the daughter of an FBI agent.
Occurs a couple of times in Robert Asprin's Myth Adventures series as a con gambit: a sexy young woman comes onto a guy, and as they're getting cozy, her companion/s burst in, claiming to be brothers, boyfriends or general protectors, protecting the girl's honour. The first time it's used, Skeeve is the mark, and the intent is to rob him. Later it comes back, with the same woman using a variant on the scheme to make the soldiers look like cads.

Live-Action TV

Inverted in the first season Cheers episode "Any Friend of Diane's," where Sam has rejected Diane's college friend Rebecca for being boring. To spare Rebecca's feelings, Diane says Sam turned her down because he and Diane are a couple (much to both Rebecca and Sam's surprise). Of course, once Diane says that, Sam and Diane use the opportunity to back-handedly insult and torture one another in front of Rebecca.
- Last episode of Cheers had Rebecca pretending to be Sam's girlfriend, while Diane's gay friend pretended to be her boyfriend.
Ross tried to do this for Rachel on Friends, but it was unwanted because Rachel was actually interested in the guy.
Orson does this for Susan in Desperate Housewives.
On Black Books there's an episode where Manny lies about Fran being his girlfriend to impress his parents.
Not sure if this counts as a subversion, but in an episode of Fringe, Olivia and Peter are at a bar looking to get information from a contact. They make their contact, and Olivia begins pumping him for information. She introduces Peter not as her boyfriend but as her brother. Later, Peter asks about the "brother" comment, to which Olivia replies something like "Trust me, it works better that way."
Alex pretends to be Elaine's boyfriend "Bill Board" on Taxi.
Rizzoli and Isles has Les Yay laden version in a scene where the titular characters dissuade a mechanic from romantically pursuing one of them by pretending to be a couple.
In Gilmore Girls, Emily and Richard invite Rory to a "Yale Party," during which she quickly realizes that she is the only young woman amidst dozens of eligible young men. When a particularly sleazy one starts to hit on her, Logan walks in.

Theatre

In On a Clear Day You Can See Forever, Edward sends off some ne'er-do-wells by pretending to be Melinda's brother, and singing "Don't Tamper With My Sister," an entire song about not going about that kind of thing (in public).

Video Games

In the game Star Ocean Till Thend Of Time, in order to get a pushy woman away from him, Fayt pretends that his friend Sophia is his girlfriend. At first she's left very shocked, but quickly catches on and goes along with it. After this all, she states that she didn't really mind being considered his girlfriend.
In the Yakuza games it's quite common for the player character to do this with hostesses they romance. Tanimura in Yakuza 4 can take a mission where he pretends to be a woman's boyfriend in order to drive away her ex for an extended period.

Web Comics

In Chapter 9 of Eerie Cuties Tia was fighting with Nina, Layla walked in, and Nina announced that's her big sister. Tia immediately "conscripted" bewildered Cessily as her "big sister".
Breadbun attempts this in Trigger Star to get away from Quiche. At first, it doesn't work, but then it works all too well. Despite this, Quiche is not entirely put off.

Western Animation

In an episode of Futurama parodying Titanic, Fry is simultaneously pretending to be Amy's boyfriend so her parents will stop harassing her about giving them grandchildren and pretending to be Leela's boyfriend to keep her from being harassed by Zap Brannigan.

Real Life

A bunch of years ago^[when?] there was some talk of a New York City mugger who would approach people (usually women) and say "walk with me." His cover story was that he was being stalked/robbed, and if his stalker saw he had a companion they would back off. Of course he got his victim into an alley or other secluded location and then robbed them.
Used in a different way by some 18th/early 19th century con men in France. The bait was an attractive teenage boy, and as soon as bait and mark were in a compromising position another accomplice would come by to barge in on the two. He would declare himself to be the unlucky younger party's older brother, or father (or simply a policeman) and threaten the mark into paying up to keep him from going to the police about this horrible affront to the young boy's decency.