Etymology
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slaughter (n.)

c. 1300, "the killing of a person, murder; the killing of large numbers of persons in battle;" mid-14c., "the killing of a cattle or sheep 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 Middle English slaught "killing, manslaughter, carnage; butchery of animals," the native cognate, from Old English sliht, sleht, slieht "stroke, slaughter, murder, death; animals for slaughter;" as in sliehtswyn "pig for killing."

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slaughter (v.)
1530s, "butcher an animal for market," from slaughter (n.). Meaning "slay wantonly, ruthlessly, or in great numbers" is from 1580s. Related: Slaughtered; slaughtering.
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slug (n.3)
"a hard blow," 1830, dialectal, of uncertain origin; perhaps related to slaughter or perhaps a secondary form of slay.
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slaughterhouse (n.)

also slaughter-house, late 14c., "place where animals are butchered for meat," from slaughter (n.) + house (n.). The Slaughter-house cases in U.S. history were 1873.

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onslaught (n.)

"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.

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manslaughter (n.)

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]
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massacre (n.)

"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."

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avicide (n.)

"slaughter of birds," 1834, from Latin avis "bird" (from PIE root *awi- "bird") + -cide.

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massacre (v.)

"to kill (many beings) indiscriminately," commonly in reference to those who are not in a condition to defend themselves, 1580s, from French massacrer "to slaughter" (16c.), from massacre (n.) "wholesale slaughter, carnage" (see massacre (n.)). Sometimes 17c.-18c. merely "to murder cruelly," without reference to number. Related: Massacred; massacring.

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blood-stained (adj.)
"stained with blood; guilty of slaughter," 1590s, from blood (n.) + past participle of stain (v.).
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