Verb
refuse to talk or stop talking; fall silent
Source: WordNetLiving in the fishbowl is hard enough without worrying about a Secret Service that can't keep mum. Eleanor Clift
There's a shop on the next corner, North Six, between Wythe and Berry? I'd like you all to go smash it up. Now, if anyone asks you who told you to do this, I want you to keep mum. It was Franz. Jack Terricloth
The children shut up when their father approached Source: Internet