clam up

Collins Concise English Dictionary © HarperCollins Publishers::

clam up verb

1. (intr, adverb) to keep or become silent or withhold information

WordReference Random House Learner's Dictionary of American English © 2025

clam¹ /klæm/   n., v., clammed, clam•ming. 
n. [countable]

1. Invertebrates a soft-bodied shellfish with two shells that close tight around it.
2. Informal Terms Informal. a secretive or silent person.
3. Slang Terms Slang. a dollar.

v. [no object]

4. to dig clams:
   They went clamming on the beaches.
5. clam up, Informal. to refuse to talk or reply: He's so shy that he clams up in public.

WordReference Random House Unabridged Dictionary of American English © 2025

clam¹  (klam), n., v., clammed, clam•ming. 
n. 

1. Invertebrates any of various bivalve mollusks, esp. certain edible species. Cf. quahog, soft-shell clam.
2. Informal Terms a secretive or silent person.
3. clamminess.
4. Slang Terms a dollar or the sum of a dollar:
   I only made 60 clams a week.

v.i. 

5. to gather or dig clams.
6. clam up, [Slang.] to refuse to talk or reply;
   refrain from talking or divulging information:
   The teacher asked who had thrown the eraser, but the class clammed up.

• short for clam-shell, i.e., bivalve with a shell that clamps. See clam², shell 1585–95

clam′like′, adj. 
clam′mer, n. 

clam²  (klam), n. 

1. British Terms clamp¹ (defs. 1–3).
2. [Mach.] (formerly) pincers.

• bef. 1000; Middle English; Old English, derivative of clamm fetter, grasp; cognate with German Klamm fetter; akin to clamp