Meaning

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

verb gather clams, by digging in the sand by the ocean

noun a piece of paper money worth one dollar

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