Past tense of clam; dug for clams or suddenly became silent and refused to speak.
From Old English 'clam' meaning to clench or grip, combined with the -ed past tense suffix. Originally referred to the action of clamming (harvesting clams), but also developed the slang meaning 'to clam up' or stop talking, from the idea of a clam's shell snapping shut.
The verb 'clam' has an amazing double life—it literally means harvesting mollusks from the ocean floor, but the phrase 'clammed up' became slang for refusing to talk because a clam's shell seals so completely. Language often borrows animal behavior to describe human stubbornness!
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