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Verb: clam up
Usage: informal
  1. Refuse to talk or stop talking; fall silent
    "The suspect clammed up when questioned by the police";
    - close up, dummy up [N. Amer, informal], shut up, belt up [Brit, informal], button up, be quiet, keep mum [informal]

Derived forms: clams up, clammed up, clamming up

Type of: forbear, refrain