umuknuti

1. (of people) to fall silent; to go quiet
   Kad je direktor ušao, svi su umukli. — When the director came in, everyone fell silent.
2. (of sounds, music) to die away; to stop
   Muzika je iznenada umukla. — The music suddenly stopped.
3. colloq. (in the imperative) shut up
   Umukni već jednom, ne mogu da te slušam! — Just shut up already, I can't listen to you!

Umukni! sounds rude; a politer request is Tiše, molim vas.