c.1300, "killing of a cattle or sheep for food, killing of a person," from a Scandinavian *slahtr, akin to Old Norse slatr "a butchering, butcher meat," slatra "to slaughter," slattr "a mowing" from Proto-Germanic *slukhtis, related to Old Norse sla "to strike" (see slay (v.)) + formative suffix (cf. laugh/laughter). Meaning "killing of a large number of persons in battle" is attested from mid-14c. Old English had slieht "stroke, slaughter, murder, death; animals for slaughter;" cf. sliehtswyn "pig for killing."
1530s, "butcher an animal for market," from slaughter (n.). Meaning "slay wantonly, ruthlessly, or in great numbers" is from 1580s. Related: Slaughtered; slaughtering.