mid-14c., " act, crime, or sin of killing another human being," in battle or not, from man (n.) + slaughter (n.). Replaced Old English mannslæht (Anglian), mannslieht (West Saxon), from slæht, slieht "act of killing" (see slay); Middle English also had man-quelling "murder, homicide" (late 14c.). Etymologically comparable to Latin homicide, but in legal use usually 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]