c. 1400, "injury, hurt, harm," from Anglo-French nusaunce, Old French nuisance "harm, wrong, damage," from past-participle stem of nuire "to harm," from Latin nocere "to hurt" (from PIE root *nek- (1) "death"). Sense has softened over time, to "anything obnoxious to a community" (bad smells, pests, eyesores), 1660s, then "source of annoyance, something personally disagreeable" (1831). Applied to persons from 1690s. As an adjective by 1889; the older adjective nuisant was always rare and now is obsolete.
"harmless, producing no ill effect, incapable of harm or mischief,"
1590s, from Latin innocuus "harmless; innocent; inoffensive," from in- "not" (see in- (1)) + nocuus "hurtful," from root of nocere "to injure, harm," from *nok-s-, suffixed form of PIE root *nek- (1) "death." Related: Innocuously; innocuousness.
"anything that causes injury, anything that harms or is likely to harm; a malady or disease; conduct contrary to standards of morals or righteousness," Old English yfel (see evil (adj.)).