slew n.1

1670s, earlier slogorne (1510s, Gavin Douglas), "battle cry," from Gaelic sluagh-ghairm "battle cry used by Scottish Highland or Irish clans," from sluagh "army, host, slew," from Celtic and Balto-Slavic *slough- "help, service." Second element is gairm "a cry" (see garrulous). The metaphoric sense of "distinctive word or phrase used by a political or other group" is attested from 1704 (as slughon). The spellings also included a fully folk-etymologized slughorn.

"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.