a shellfish that can be eaten. It has a shell in two parts that can open and close. clam chowder/soup The moment I mentioned the children he closed up like a clam (= became silent and refused to discuss the topic). See related entries: Invertebrates Word Origin early 16th cent.: apparently from earlier clam ‘a clamp’, from Old English clam, clamm ‘a bond or bondage’, of Germanic origin; related to Dutch klemme, German Klemme, also to clamp.