close up
clam up
dummy up
shut up *
belt up
button up
be quiet *
keep mum

The guy I talked to knows how to keep mum.

It don't make any difference what happens, we got to keep mum.

He is also not allowed to keep mum when someone asks him a question.

But if you stay, keep mum and all that - they won't know anything.

Better to play it safe, he thought, and keep mum.

I'd be sure of my dough if I kept mum.

"Well, just remember to keep mum if you do get the chance," she said.

Company policy, which he does his best to observe, is to keep mum on politics.

Talk frankly, and I'll give you my word to keep mum.

I'm telling you this, so as you'll know to keep mum.