Shut yer gob!

...or, if you are French, ta gueule!

Urban Dictionary says this is:
a French slang/argot expression: to demand silence in a violent or immediate way: 1. shut up; shut your trap/gob/hole, etcetera. 2. While sufficient when used alone, is frequently combined with a descriptive noun that is usually insulting, vulgar or rude (often all three).

...and helpfully gives some examples of such nouns (and a coarse québécois phrase which was new to me).

Widely useful as the vulgar gueule can be—gueuleton, dégueulasse, amuse-gueule (better than the prissy amuse-bouche)—it crops up less often in French than gob does in English. The OED lists five distinct nouns , of which the oldest, meaning mass or lump, was first recorded in 1382, and three verbs. Also, of course, there are dozens of compounds (gob-stopper, gobshite, gobsmacked...).

The one we are most familiar with means mouth, and was first recorded in 1550: S.T, xxv: Quhair thair gobbis wer ungeird, Thay gat upon the gammis. I have no idea what that means, though it sounds like the first two lines of a rude poem.

I had not intended to get involved with gueules and gobs; I just got sidetracked, as one often does when treading the miry paths of the OED. Having recently posted pieces about some huge meals which might well be considered spew-making or pukogenic, I started idly looking at synonyms for vomit, and one thing led to another. Anyway, the really interesting thing I found—well, interesting to me at any rate—was that there is a French word dégobiller meaning to throw up.

As one of the meanings of gob is to spit then there must surely be a connection here. With the help of Céline, I established that etymologically the French word does not stem from the English. And gob meaning spit, according to the OED, is: Of obscure origin; possibly Gael. and Irish; nothing about a French connection..

But, if we look at gob meaning a mouthful, lump, we find the etymology is: OF. gobe, goube (mod.F. gobbe). In mod.F. only in the special senses of a food-ball for poisoning dogs, feeding poultry, etc., and a concretion found in the stomachs of sheep), related to the vb. gober to swallow.

Leaving aside the question of whether you can really poison dogs and feed poultry with the same gobbe, this shows clearly that the OED etymology for gob (=spit) is probably wrong: we got it from the French.

So there.