I Know Karate

A scene in which a martial artist warns an antagonist that he knows martial arts before kicking the guy's ass. Bonus points for the martial artist assuming an outlandish Asskicking Pose beforehand. The antagonist will always laugh off the warnings and be defeated in humiliating fashion. These scenes were popular when martial arts films started getting attention in the west. The martial artist will almost always study an Asian style, but the trope can still hold true with any named system of self-defence.

This trope is the inversion of Good Old Fisticuffs, in which a haughty martial artist gets defeated by plain, no-nonsense asskicking.

It's pretty much a Dead Horse Trope now that Asian martial arts films and choreography have reached a saturation point in western culture. Ironically, the trope might have come full circle due to the rising popularity of Mixed Martial Arts over most classic Asian martial arts styles.

You might be looking for Suddenly Always Knew That, which used to be called "I Know Kung Fu".

"Mitsuki: *takes a fighting stance* Y-you'd better not take me lightly! I may not look like it, but I've been practicing Karate since I was a ki- Sasaki: *casually backhands Mitsuki, sending him flying*"
 * In the first episode of season 2 of Lincoln Heights, Cassie Sutton completely demolishes a would be racist attacker, much to the surprise of her boyfriend Charles. Her father taught the whole family defensive tactics after the hellish season 1 the family went through. Oddly enough this is the only time any of the Sutton kids displayed any martial arts abilities despite there being numerous occasions where they are needed.
 * 'Armageddon Trilogy' book by Robert Rankin. This exact line is said repeatedly by Elvis (yes, that Elvis). In the books Elvis constantly makes reference to his character from the movie Roustabout, in which he knew Karate (it goes with the 'sickle)
 * Also, subverted by the phrase "I must warn you that I am an exponent of Dimac, the world's most lethal martial art, and can maim or disfigure you with the mere pressure of a fingertip", because this phrase always causes the opponent to back down. Inevitably.
 * It's worth noting that in real life, Elvis was an 8th degree black belt in Kenpo Karate.
 * Kung Fu did this, of course.
 * Knight Rider did it at least once. Given how tall David Hasselhoff is, you can imagine just how big the local color had to be to carry this off.
 * Subverted in The Return of Sherlock Holmes: Holmes warns the bruiser, "I know judo." The bruiser knew karate - and knew it better than Holmes did judo.
 * In Doctor Who, the Third Doctor was a skilled practitioner of Venusian Aikido and, despite being grey headed and somewhat frail looking, had little difficulty in physical altercations, even when facing off against several opponents.
 * Subverted in an episode of Alias in which Quentin Tarantino starred. When Sydney attempts to beat him down with her martial arts skills, Tarantino susses out her style and responds with, "That's the problem with you kickboxers -- none of you can take a punch." And proceeds to knock her silly with her own skillset. She later acknowledges she does need to learn to take a punch...and kicks him in the head when he's down. (Also an aversion of Wouldn't Hit a Girl.)
 * Subverted in I'm Gonna Git You Sucka when the group's martial arts expert is surrounded by cops he says something along the lines of "You dare to challenge me?" "A master of karate, kung fu, jujitsu and all this other shit you've never heard of?" "Ha ha ha!" The cops respond by opening fire on him.
 * And again later, when one of the heroes and bad guys square off against one another minus their guns, one of them hesitates and says, "I don't know any kung fu!" "Neither do I," says the other. "Want to fake it?" They then do just that.
 * Parodied and subverted in Romeo Must Die when big black gangster Maurice corners small Chinese man Han. Maurice does the crane stance because "Now, you know you ain't the only one that knows some shit," and Han (or as we call him, Jet Li, the last person in the world who'd be impressed by the crane stance) immediately kicks out Maurice's supporting knee.
 * Parodied and subverted in Friends, where Ross is learning (comically overpronounced) karate. He claims to have supernatural awareness called "unagi" (actually a Japanese freshwater eel) but gets ambushed at the end of the episode.
 * Subverted in Gate Keepers anime: Kageyama Reiji did this complete with throwing his jacket to the air for a good montage, even without the lines. When the jacket drops back on his hand, the thugs seemed to be hurt, until he himself collapsed. Though,
 * That guy with the glasses in The Devil's Brigade, which he apparently wears only so he can have The Glasses Come Off.
 * Parodied in Space Quest 5: The Next Mutation, where Roger Wilco warns the Big Bad "I know Kung Fu, Karate, Tae Kwon Do, Judo, and several other Chinese words!"
 * An episode of Scooby Doo, Where Are You? has Shaggy and Scooby trying to bluff a Chinese ghost with the line "I know judo, chop suey, and Chinese checkers!" This being Shaggy and Scooby we're talking about, they later escape from the ghost's henchmen by serving them chocolate chop suey (with liver a la mode).
 * Subverted in The Adventures of Jimmy Neutron, where Cindy is captured and subsequently manipulated by Eustace Strych despite having yelled "I Know Karate"!
 * Subverted sort of in Worst by one of the villains.

"Gabe: You leave me no choice. Now, I have to do karate to you. Clerk: Do you even know karate? Gabe: I know techniques that have certain elements which have been described--by some--as "karate-like"."
 * Parodied in Penny Arcade. For extra hilarity, the guy he's talking to is Asian.

"Ella: I know the Japanese art of... origami! Bandit: Paper folding? Ella: Oh... I was hoping you wouldn't know what that was."
 * In The Film of the Series of Wild Wild West, a Mook attempts to intimidate Will Smith with a series of moves he "learnt from a Chinaman". Smith then whacks him with a shovel; "I just made that up".
 * Subverted in the Ella Enchanted film.

"Bobbie: Listen skinny, before you start talking tough, I'd better warn you I've got a black belt in karate. So why don't you get out of here quietly, while you still got some teeth left in that ugly face? [Foxy knocks her down with a barstool] Foxy Brown: And I've got my black belt in barstools!"
 * This memorable exchange in Foxy Brown:

"Anton: I know Tai-chi! Paw-paw-paw! I know Taikwondo! Paw-paw-paw! And I know Tyrone... yeah, he taught me all of that."
 * In an episode of The Flintstones, Fred tries this line, only for his opponent to go, "Who is that?" before beating him up.
 * In the film adaptation of Rising Sun, a bouncer warns Sean Connery that he's a black belt. Connery takes him out with a throat strike.
 * In Living Color had Damon Wayans' character Anton the bum use this to try to get into the army (for the free food, of course)

"Cassandra:First, you have to get through me. Kent:What do you know? Karate? Cassandra:I know Dow. Kent:Dow? What's Dow? Cassandra:Dow is the chemical company that makes mace. She pulls out a can of mace and sprays them."
 * Subverted at one point with Spenser. He is questioning a female gay rights activist, and the activist bristles when Spenser inevitably goes into Sarcasm Mode. Her (also female) bodyguard decides the interrogation has gone on long enough and tells Spenser to back off or she'll use her karate to "kick his nuts into his ass". He tells her to come on, then she kicks him in the balls. She is about to attempt using martial arts on him, but Spenser is able to fight through the pain and proceeds to drop her with one punch. After she regains consciousness he instructs her on how a kick to the balls are all well and good, but if the big guy you're fighting has enough of a weight-and-strength advantage over you (and Spenser towered over her), and knows how to fight himself (Spenser really, really does), all the kicks and martial arts in the world will not help you.
 * There's another moment in Stardust when a guy he's interviewing gets pissed, and has his bodyguard put on a wondrous display of karate skill by beating the hell out of a heavy bag, in order to intimidate Spenser into backing down. Spenser pulls out a revolver and shoots the bag.
 * Jade in Jackie Chan Adventures threatened a bully with the 'ancient art of butt whoop'. Considering she's the niece of Jackie Chan, you can guess how this ends up.
 * She gets into trouble for thoroughly beating up the bully, who later comes back asking her to teach him. Instead, she teaches him self-control (something she herself lacks), and he is later shown breaking up a fight.
 * Roger Moore (As Himself) attempts to threaten a thug like this in The Cannonball Run, saying: "I must warn you, I am Roger Moore" before trying an obviously fake martial arts move much like the ones he was using in his Bond movies. It fails to work.
 * In Pre Crisis Superman comics, it was established that Krypton had martial arts. Faora of the Phantom Zone knew Horo-Kanu, with which she was able to knock Superman around pretty effectively. And Lois studied the art of Klurkor in the bottle city of Kandor, and got to use it from time to time.
 * Sir Rodney from The Wizard of Id once tried to use this trope by telling a larger attacker that he had a black belt. Turned out the guy had the black suspenders.
 * A Kim Possible fan fiction inverted this, playing out an alternative to one episode where the large football player was warned that the ninja knew martial arts. The martial artist naturally won. In the fan fiction though, it was turned around with the football player being a boxer, and succeeding.
 * In an episode of Garfield and Friends, Garfield unwittingly angered a bunch of dogs and tried to discourage them from harming him by telling several lies. One of them was that cats knew karate. It didn't work.
 * In a Garfield strip, a mailman says this to Garfield. Garfield beats him up and notes: "And I know fast and furious."
 * Jose Carioca once tried to scare a bunch of criminals by claiming to be a good capoeira fighter, only to learn one of the criminals was a master of that art. He then claimed to know karate and was told another crimimal of the bunch was a black belt.
 * Punkin Puss And Mushmouse: Mushmouse once practiced Judo with a dummy and then tried to fight Punkin Puss and warned him about knowing judo. However, as Punkin Puss explained after defeating Mushmouse, he wasn't a dummy.
 * In Recess: School's Out, TJ attempts to scare off the bad guys by yelling that he's "a Black Belt in Origami". It doesn't work.
 * In Dutch, an adolescent warns Dutch that he's a "high brown belt," then proceeds to kick his ass. Later, Dutch tries to pit his self-described "all-American streetfighting" against the kid's martial arts and loses again. He does, however, teach the kid how to throw a wicked haymaker.
 * Parodied in One Crazy Summer.