What does "clam up" mean?

To become silent; to stop talking.

What is a synonym for "clam up"?

Common synonyms include: be quiet, belt up, button up, close up, dummy up, freeze up.

verb, slang

verb, slang ·Moderate ·College level

Verb

1
To become silent; to stop talking. informal, intransitive
"Maybe he … or she … was a witness who refused to clam up … a whistle blower …"
2
refuse to talk or stop talking; fall silent wordnet
3
To make or become clammy. ambitransitive

"Maybe he … or she … was a witness who refused to clam up … a whistle blower …"

By analogy with a clam closing itself. Compare clamshell (“mouth”), clam (“mouth”).