1
general::
Phrase(s): Mum’s the word.
Fig. a pledge not to reveal a secret or to tell about something or someone. • "Mum’s the word," said Jane to ease Mary’s mind about her secret.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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."
American Heritage Idioms