"cast off" (as the skin of a snake or other animal), 1720, originally of diseased tissue, from M.E. noun meaning the skin thus cast off (c.1300), probably related to O.S. sluk "skin of a snake," M.H.G. sluch "snakeskin," M.L.G. slu "husk, peel, skin," from P.Gmc. *sluk-.