c.1440, from O.N. bulki "a heap, ship's cargo," thus "goods loaded loose," perhaps lit. "rolled-up load," from P.Gmc. *bul-, from PIE *bhel- "to blow, swell." Meaning extended by confusion with obs. bouk "belly" (from O.E. buc, from P.Gmc. *bukaz, from PIE root meaning "to swell"), which led to sense of "size," first attested c.1449.