clam up

• (idiomatic) shut up like a clam

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

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clam up (third-person singular simple present clams up, present participle clamming up, simple past and past participle clammed up)

1. (intransitive, informal) To become silent; to stop talking, to shut up.
   • 1986, Paul Chadwick, Concrete: Under the Desert Stars, Dark Horse Books:

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

2. (transitive, intransitive) To make or become clammy.

• shut up

• talk