Green’s Dictionary of Slang

stumm and crum adj.

[ext. of shtoom adj.]

extremely quiet, ‘silent as the grave’.

[UK]D. Powis Signs of Crime 203: Stumm Silent. Pronounced Schtumm. ‘Keep stumm!’ is an urgent command – ‘Stop talking!’ (Yiddish.); Stumm and crum Emphatic version of ‘stumm’.