clam (v.) Look up clam at Dictionary.com
"to dig for clams," 1630s, American English, from clam (n.). Clam up "be quiet" is 1916, American English, but clam was used in this sense as an interjection mid-14c.
clam (n.) Look up clam at Dictionary.com
bivalve mollusk, c. 1500, in clam-shell, originally Scottish, apparently a particular use from Middle English clam "pincers, vice, clamp" (late 14c.), from Old English clamm "bond, fetter, grip, grasp," from Proto-Germanic *klam- "to press or squeeze together" (source also of Old High German klamma "cramp, fetter, constriction," German Klamm "a constriction"), possibly from a PIE *glem- or *glom- "contain, embrace" (see glebe). If this is right then the original reference is to the shell. Clam-chowder attested from 1822. To be happy as a clam is from 1833, but the earliest uses do not elaborate on the notion behind it, unless it be self-containment.