www.alphadictionary.com

A recent addition, meaning silent. From Yiddish and German.

I saw this in an article in The Guardian. Here it is in context:

Russia’s president keeps understandably schtum about his “special military operation”.

I guess everyone's keeping mum on comments here.

German "stumm" is "mute", or "dumb" as in (now not politically correct) "deaf and dumb". A slightly different connotation.