valve, flap (valve)
Je sais pas si on pourra lui fermer le clapet.
I don't know if we'll be able to shut him up.
— Blow Out (1981)
Je vous saurais bien gré de fermer votre clapet !
I would appreciate it if you would shut your mouth!
Je vais lui faire fermer son clapet.
I'll make him shut his mouth.
Il faut que tu fermes ton clapet.
You need to shut your mouth.