slaughter (n.)

Middle English slēn, from Old English slean "to smite, strike, beat," also "to kill with a weapon, slaughter" (class VI strong verb; past tense sloh, slog, past participle slagen), from Proto-Germanic *slahanan "to hit" (source also of Old Norse and Old Frisian sla, Danish slaa, Middle Dutch slaen, Dutch slaan, Old High German slahan, German schlagen, Gothic slahan "to strike"). The Germanic words are said to be from PIE root *slak- "to strike" (source also of Middle Irish past participle slactha "struck," slacc "sword"), but, given certain phonetic difficulties and that the only cognates are Celtic, Boutkan says the evidences "point to a North European substratum word."

The verb slēn displays many nondialectal stem variants because of phonological changes and analogical influences both within its own paradigm and from other strong verbs. [Middle English Compendium]

Modern German cognate schlagen maintains the original sense of "to strike." Meaning "overwhelm with delight" (mid-14c.) preserves one of the wide range of meanings the word once had, including, in Old English, "stamp (coins); forge (weapons); throw, cast; pitch (a tent), to sting (of a snake); to dash, rush, come quickly; play (the harp); gain by conquest."

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]