Slang term used mainly in London to mean quiet or silent. Pronounced "shtoom".
Keep shtumm and we won't get caught.
Shtum is closely related to the German word "stumm". Both wordsmean not to say anything. There is no correlation between "dumb" and shtum, as dumb (or dumm in German" has it roots in "not knowing".
The witness remained shtum
adjective: to keep quiet, with regard to not talking about something.
Used throughout British Isles
You better keep shtum about that before the whole world knows!
Comes from the German word for 'dumb' (incapable of speaking), though it is spelt 'stum' in German. Spelling in English somewhat ambiguous.
"Keep stum or I'll chop your facking balls off sunshine!"