shtum

(also schtum)
• informal

ADJECTIVE
Silent; non-communicative:

He kept shtum about the fact that he was sent down for fraud
He kept shtum and didn't embarrass her once for the following 11 years and, instead, contented himself with being an almost mute second fiddle to a woman of whom he was inordinately proud.
I kept shtum that I had just completed a Politics degree under an America-obsessed Prof.
I know you complain about me posting way too much for you to read, so, other than significant breaking news, I will keep shtum until at least Friday afternoon.

VERB (shtums, shtumming, shtummed)
Be or become quiet and non-communicative:

You start to say something and then just when it’s getting interesting you shtum up
This source was schtummed when Julia posted a scathing rebuke on the thread, really very angry.
The possible risk being that she schtums and won't tell me the truth or just laughs in my face when I bring it up because it's apparently trivial.
It's when it gets to the literature type of questions which totally schtums me, I mean I haven't got a clue.

Origin
1950s: Yiddish, from German stumm.