"Be Quiet!" Nudge

You and your buddies are planning a prank, share a secret that the new guy doesn't know, or just plain has something to hide. Except one of them, usually The Ditz or a Cloudcuckoolander, can't help giggling or indulging in snarky remarks about the situation at hand. Or is about to tell too much, to reveal a secret or to let slip some important tidbit that better should stay unsaid. Or to express surprise at some statement, thus potentially ruining any chance of making a lie believable. Or just to comment about something at the inapropriate time and place, typically making a social situation awkward.

Cue the "Be Quiet!" Nudge in some form of Slapstick violence upon the offender: an elbow/punch to the gut, a kick under the table, a stomp to the foot, a Dope Slap, etc.

If the offender is exceptionally clueless, expect them to complain: "Ow! What was that for, and what does 'ixnay' mean?" Thus eliciting another, harder, punch, stomp or kick. May induce Agony of the Feet.

Contrast with Footsie Under the Table.

Examples of "Be Quiet!" Nudge include:

Advertising

There's an ad campaign for Chef Boyardee which is this same idea, except the harm comes to a display pyramid of cans instead of the woman who won't stop talking about the full serving of vegetables in the product.

Anime and Manga

Happens often on Sailor Moon with Usagi.
In Saint Beast, Goh applies his fist to Gai's head whenever he says something tactless or gets too worked up. Gai never understands what the problem is.

Comic Books

Happens in Elf Quest: Skywise nudges Cutter to shut him up when he's about to drunkenly blurt out the location of Sorrow's End to Picknose the troll.
Tintin slams his heel into Haddock's foot to stop him from revealing information in The Calculus Affair.

Film

In something of a role reversal, The Ditz Miguel does this to The Smart Guy Tulio in The Road to El Dorado.
Tweedledee nudges Tweedledum -- or was it the other way around?—in the Tim Burton rendition of Alice in Wonderland to keep him from blurting out that the Red Queen's new favorite, Um from Umbridge, was really the fugitive Alice.
Clue, when the policeman inspects the body of the Motorist and says he's in no shape to drive:

Miss Scarlet: Oh, we'll-- we'll-- we'll get him a car!
Professor Plum: A long black car!
Miss Scarlet: (punches him in the stomach) A limousine!

In The Great Race, Leslie administers the elbow-to-ribs version to Hezekiah when Hezekiah laughs too long at a joke the Crown Prince made.
In His Girl Friday, Hildy Johnson keeps kicking Walter Burns under the table as he repeatedly insults her dull new fiancé, Bruce Baldwin (who doesn't notice). She ends up kicking the waiter.
In Spider-Man 2, Aunt May kicks Peter under the table when he responds with surprise to her announcement that she's resumed giving piano lessons as she tries to get a loan at the bank, but she misses and gets the loan officer instead.
The same subversion happens in Good Burger, when Roxanne is trying to get Ed to spill the beans about the secret sauce. Before Ed can talk, Dexter kicks under the table, hitting Roxanne's foot.

Literature

Happens in one Animorphs novel, where Ax is narrating. He's posing as Jake and is kicked by Cassie before he blows his cover ("Why has your father removed the hair from the top of his head?")
Happens several times in Harry Potter. It's noted that it's a lot more discreet when you're wearing robes rather than Muggle trousers.
Happens a lot in Piers Anthony's works, most often in the form of a kick.
Done by Ridcully in Soul Music, to Ponder, who can't understand why he should keep his mouth shut about their "sound-trap" in front of sausage-inna-bun vendor turned music producer C.M.O.T. Dibbler.
In The Elenium, Elhana attempts this with her husband. Of course, since he's wearing plate armour, all she gets out of it is a bruised elbow.
In one of the Dragon Jousters novels, Ari is about to say something that would ruin his relationship with Nofret. Kiron kicks Ari before foot-in-mouth ensues.
Happens several times in Septimus Heap, including an example from Syren where Jenna yanks at Lucy Gringe's cloak to tell her to be quiet while they are deciding upon how to take the ship Cerys' back.

Live-Action TV

Cheers used this to comedic effect often.
Ditto Frasier
NCIS:
- Gibbs prefers the Gibbs Slap.
- Kate typically prefers the elbow-to-the-ribs whenever Tony is, well, being Tony.
Used in a Thanksgiving episode of Home Improvement. Twice. After Al has explained—at great length—why he's thankful for his girlfriend:

Brad: I'm thankful that Al's done being thankful! (Ow!)
Randy: And I'm thankful that Brad said that instead of me. (Ow!)

Used a handful of times on The Nanny, often with an addition to it.
- Once, Fran continually corrects Max by smacking his hand with a spoon. When he finally gets it right, she smack a water glass, creating a game-show "Correct!" ding sound.
- Used again on Max when he is trying to comfort Fran after another failed date saying she didn't need a man. Sylvia silences him.

Sylvia: (holding a carving fork) Oh, I'm sorry. Did this fork accidentally puncture your tuckus?

Puppet Shows

Used in Les Guignols de l'info during an interview of Roselyne Bachelot and Xavier Bertrand (former and actual Health Ministers, respectively). Roselyne, who's very much The Ditz, is about to tell everything about the scandalous links between the French government and the "Big Pharma", forcing Xavier to stomp on her foot. Twice. And when it's not enough to silence her, he upgrades to a "Be Quiet!" Shovel to the Face.

Web Comics

Steve from Questionable Content uses this as a signal to Marten to leave Steve alone with his lady-friend of the evening.
Happens a few times to Belkar in The Order of the Stick:
- In strip #247, Haley is struck with aphasia and so can't talk Belkar into shutting it as he's about to tell Elan her feelings. However, a boot to the face is enough to convey the message more clearly.

Belkar: Objection noted.

- Also in strip #379, Roy kicks Belkar so that the halfling doesn't join the already-embarrassing conversation.
Bridget shuts Alphonse up in this page of Far Out There, lest he should give away the fact that they were the ones who hired Skye. Of course, since neither of them can actually talk, one wonders what she's afraid of.
Dhur the barbarian to Becky Sourbottom in Meat Shield, with Agony of the Feet bonus.
In Exterminatus Now, Virus elbows Eastwood while they're trying to hire mercenaries, as it wouldn't be a good idea to dwell on what happened to their previous hired guns. Those poor sons of—OW!
Girl Genius
- Oggie was silenced repeatedly. [1] [2] [3]
- Krosp had to bite Dimo. Knowing the Jägers, he could miss a lesser signal if distracted.

Western Animation

Done a couple of times in Avatar: The Last Airbender.
Done in Foster's Home for Imaginary Friends, except the recipient is doing the whole Totem Pole Trench routine, so is blissfully unaware of "his" foot being kicked.
Wakfu, season 2 episode 8: After Sadlygrove blurts out that he'd already seen Evangelyne naked, Eva, while denying, casually chokes Grovy with an arm around his neck to stop him from adding a word.
In The Problem Solverz episode "Badcat", the gang is in disguise to sneak into Bad Cat's casino. When they're allowed in, Roba keeps saying how he feels so loved and wanted, which causes Horace to shush him in order to stay incognito.
Codename: Kids Next Door: Numbuh Five is usually the one to elbow her teammates when they need to shut it, like with Numbuh Four at the end of "Operation: R.O.B.B.E.R.S.", or Numbuh Three in "Operation: C.A.K.E.D.-F.I.V.E.". Note that it hardly ever works on the latter: if you truly want Kuki to shut up, you need to gag her.
My Little Pony: Friendship Is Magic: Twilight Sparkle has to give one to Spike in "Hurricane Fluttershy".
On Regular Show, Mordecai does this to Rigby all the time.