n. (informal)
25B6; adjective he was keeping mum : SILENT, quiet, mute, dumb, tight-lipped, unforthcoming, reticent; archaic mumchance.
25A0; mum's the word SAY NOTHING, keep quiet, don't breathe a word, don't tell a soul, keep it secret, keep it to yourself, keep it under your hat; informal don't let on, keep shtum, don't let the cat out of the bag.