any of various burrowing bivalve molluscs of the genera Mya, Venus, etc. Many species, such as the quahog and soft-shell clam, are edible and Tridacna gigas is the largest known bivalve, nearly 1.5 metres long
c.1500, "bivalve mollusk," originally Scottish, from M.E. clam "pincers, vice, clamp," from O.E. clamm "bond, fetter," from P.Gmc. *klam-.Clambake is from 1835. Clam up is 1916, Amer.Eng., but clam was used in this sense as an interjection c.1350.