From WordNet (r) 3.0 (2006):
mum
adj 1: failing to speak or communicate etc when expected to;
"the witness remained silent" [syn: mum, silent]
n 1: of China [syn: florist's chrysanthemum, florists'
chrysanthemum, mum, Dendranthema grandifloruom,
Chrysanthemum morifolium]
2: informal terms for a mother [syn: ma, mama, mamma,
mom, momma, mommy, mammy, mum, mummy]
3: secrecy; "mum's the word"