keep mum

Từ điển WordNet v3.1 - WordNet Dictionary

verb

refuse to talk or stop talking;
fall silent
- The children shut up when their father approached
• Syn:
close up, clam up, dummy up, shut up, belt up,
button up, be quiet
• Ant:
open up (for: close up)
• Verb Frames:
- Somebody ----s