A word used mainly by school students to warn of approaching teachers/authority figures. More effictive that saying "Shhhh" or "Shut-up, teacher", which tends to raise supsicion.
Used to hault a potentially punishable verbal tirade, or merely to instill caution.
Student 1: My God, that new guy is such a goddamn cun-
Student 2: Mush!
Student 1: Good Afternoon, sir...