c. 1300, "the killing of a person, murder; the killing of large numbers of persons in battle;" mid-14c., "the killing of cattle or sheep or other animals for food;" from a Scandinavian source akin to Old Norse slatr "a butchering, butcher meat," slatra "to slaughter," slattr "a mowing," related to Old Norse sla "to strike," from Proto-Germanic *slagan- (see slay (v.)).
The form was perhaps influenced by obsolete slaught "killing, manslaughter, carnage; butchery of animals," the native cognate, which is from Old English sliht, sleht, slieht "stroke, slaughter, murder, death; animals for slaughter;" as in sliehtswyn "pig for killing." The Elizabethans had an adjective slaughterous.
1530s, "butcher an animal for market," from slaughter (n.). Meaning "slay wantonly, ruthlessly, or in great numbers" is from 1580s. Related: Slaughtered; slaughtering.
"killing, manslaughter, carnage; butchery of animals," now obsolete (OED's last entry is c 1610), the native cognate of slaughter (q.v.). From Old English sliht, sleht, slieht "stroke, slaughter, murder, death; animals for slaughter;" as in sliehtswyn "pig for killing." Cognate with Old Saxon slahta, Old Frisian slaehte, Old High German slahta, German Schlacht "battle."
"attack, aggression, assault," 1620s, anslaight, apparently somehow from or on analogy of Dutch aanslag "attack," from Middle Dutch aenslach, from aen "on" (see on) + slach "blow," related to slaen "slay." Early sources say the word was from Dutch, but the forms do not correspond well. The spelling would have been influenced by English slaught (n.) "slaughter," from Old English sleaht (see slaughter (n.)), but this word, obsolete since c. 1400, can't be the source of the modern one unless the record is imperfect. There is no record of onslaught in 18c.; apparently the word was revived by Scott.
early 14c., " act, crime, or sin of killing another human being," in battle or not, from man (n.) + slaughter (n.). It gradually displaced manslaught, the earlier word, from Old English manslæht (Anglian), manslieht (West Saxon), from slæht, slieht "act of killing" (see slay (v.)). Middle English also had man-quelling "murder, homicide" (late 14c.), and slaughter-man (late 14c.), "an executioner; a butcher."
Etymologically it is comparable to Latin homicide, but in legal use usually it is distinguished from murder and restricted to "simple homicide, unlawful killing of another without malice either express or implied."
Manslaughter differs from murder in not proceeding from malice prepense or deliberate, which is essential to constitute murder. It differs from excusable homicide, being done in consequence of some unlawful act, whereas excusable homicide happens in consequence of misadventure. Manslaughter has been distinguished as voluntary, where the killing was intentional in a sudden heat or passion without previous malice; and involuntary, where it was not intentional, but the slayer was at the time engaged in an unlawful act less than a felony, or doing a lawful act in an unlawful manner. [Century Dictionary]
"unnecessary, indiscriminate killing of human beings," sometimes also applied to wholesale slaughter of animals, 1580s, from French massacre "wholesale slaughter, carnage," from Old French macacre, macecle "slaughterhouse; butchery, slaughter," which is of unknown origin; perhaps related to Latin macellum "provisions store, butcher shop," which probably is related to mactāre "to kill, slaughter."
"great destruction by bloody violence, massacre," c. 1600, from French carnage (16c.), from Old Italian carnaggio "slaughter, murder," from Medieval Latin carnaticum "flesh," from Latin carnaticum "slaughter of animals," from carnem (nominative caro) "flesh," originally "a piece of flesh" (from PIE root *sker- (1) "to cut"). In English it has been always used more often of the slaughter of men than beasts. Southey (1795) tried to make a verb of it.