Fermez vos gueules is everyday blunt French slang, with gueule being a crude word for mouth usually reserved for animals or insults, making the plural command sound even harsher than a polite taisez-vous. Short shouted phrases like this spread easily on soundboards and in meme edits because the meaning survives translation and the delivery is instantly readable. It is now used less as a literal insult and more as a stock silencing reaction alongside other short meme shouts, dropped in whenever a chat, stream or video gets too loud.