clam up

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, dummy up, shut up, belt up, button up,
be quiet, keep mum
• Ant:
open up (for: close up)
• Verb Frames:
- Somebody ----s