Schtum/shtoom

Variously spelt shtum, shtoom or stumm means to keep quiet and say nothing. It looks like another of those words from Yiddish but a more likely origin is the British underworld or British prison slang from the mid-20th century because of this first citation from a book called Bang to rights: an Account of Prison Life (1958) by Frank Norman. “I think it’s much better to keep shtoom. You can always shtoom up if any of the screws are earhole-ing.”