Meaning

noun burrowing marine mollusk living on sand or mud; the shell closes with viselike firmness

noun a piece of paper money worth one dollar

noun flesh of either hard-shell or soft-shell clams

originFrom Middle English clam (“pincers, vice, clamp”), from Old English clam (“bond, fetter, grip, grasp”), from Proto-West Germanic *klammjan (“press, squeeze together”). The sense “dollar” may allude to wampum. The sense "Scientologist" alludes to the Scientologist belief that human thetans (souls) previously inhabited clams.

Etymology from Wiktionary, CC BY-SA 3.0