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.

   Synonyms: shut up; see also Thesaurus:be quiet

   • 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 …

   • 1993, “Matt Foley: A Scary Story on Halloween”, in Saturday Night Live‎^[1], season 19, spoken by Matt Foley (Chris Farley):

     Teenagers' father. Alright, Matt, I think you've made your point.
     Matt Foley: Dad, I sincerely wish you'd do everyone a great service, and clam up!

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

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