slough n.1

"muddy place in a road or way, mudhole, swamp, deep quagmire," Middle English slough, from Old English sloh "soft, muddy ground," a word of uncertain origin. Compare Middle Low German sloch "muddy place," Middle High German sluoche "ditch." Figurative use (in reference to moral sunkenness) is attested from mid-13c.; Bunyan's "Slough of Despond" is 1678.

1720, intransitive, "come off as slough;" 1762, transitive, "to cast off" (as the skin of a snake or other animal), from the Middle English noun slough "shed skin of a snake" (see slough (n.)). Originally of diseased tissue. Related: Sloughed; sloughing.

"cast-off skin" (of a snake or other animal that normally sheds or molts), early 14c., slughe, slouh, which is probably related to Old Saxon sluk "skin of a snake," Middle High German sluch "snakeskin, wine-skin," Middle Low German slu "husk, peel, skin," German Schlauch "wine-skin;" from Proto-Germanic *sluk-, which is of uncertain origin, perhaps from PIE root *sleug- "to glide." By 1510s as "mass or layer of dead tissue around a wound, etc."