[muhm] 
| 1. | silent; not saying a word: to keep mum. |
| 2. | say nothing! be silent! |
| 3. | mum's the word, do not reveal what you know (about something); keep silent: Mum's the word, or the surprise party won't be a surprise. |
| mum 1 (mŭm) adj. Not verbalizing; silent. interj. Used as a command to stop speaking. [Middle English, perhaps imitative of closing one's lips.] |
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."