adj. 1 silent, mute, close-mouthed, quiet, tight-lipped She swore she would keep mum about where the money was hidden
n. 2 Mum's the word. Don't tell a soul, Keep silent, Keep secret, Keep quiet, Keep (something) to oneself, Keep (something) under one's hat, Say nothing, Tell no-one, Play dumb He told who stole the bell, but mum's the word