From Longman Dictionary of Contemporary Englishkeep mumkeep muminformalTELL A SECRET to not tell anyone about a secret → mum
Examples from the Corpus
keep mum• In politics, as in life, truth hurts.Better to keep mum.• The grinning skull on the back of the mystery woman's jacket is keeping mum.• So, darling, keep mum about where we are.• Far right: Septimus keeps Mum and Dad awake.• That obviously includes me, so your mum's got to keep mum, chick. 