|chiefly (Brit) an informal word for mother|
|[C19: a child's word]|
mum's the word
Say nothing about this, it's a secret, as in Mum's the word on tonight's surprise party. This expression dates from about 1700, but mum, meaning "silence," is much older. In 2 Henry VI (1:2) Shakespeare wrote, "Seal up your lips, and give no words but mum."