If someone asked you to keep schtum, it would mean that they didn’t want you to say anything as it might get you into trouble. According to Phrases.org.uk schtum possibly comes from the German word stumm meaning silent.
As an adjective it means ‘silent ‘and ‘non-communicative’
Example: I asked her to keep schtum about my lottery win.
Verb (schtum, schummed, schtumming)
As a verb it means ‘be or become quiet and non-communicative’.
Example: Jean schtummed up when I mentioned the fraudulent activity on the account.
Variations of the way it’s spelt:
If you use the word, how do you spell it?