This is the 4396th most frequent French word.
gueule
Mouth or face (informal, often for animals or slang for humans).
Here, "gueule" is used figuratively to refer to someone's tendency to talk excessively or boast in an informal context.
Il a une grande gueule.
He talks a lot (literally: he has a big mouth).
In this usage, "gueule" is combined with "ferme" to form a blunt and impolite command, often demanding silence. "Gueule" here roughly translates to "mouth" in a rude manner.
Ferme ta gueule !
Shut up!
Here, "gueule" refers to the literal mouth of an animal. The term "gueule" is commonly used to describe the mouths of animals, differentiating them from humans' "bouche".