h₂ney-
“to be hostile, harm”Source of Latin nocēre, English nuisance, innocent, noxious
Root for hostility or harm, yielding Latin nocēre (to harm) and English innocent, noxious, nuisance.
Discussion
The PIE root *h₂ney- (to be hostile, to harm, to kill) produced one of the darkest word families in the IE vocabulary — connecting hostility to death, corpses, and the underworld.
Latin nocēre (to harm, to injure) continues the root and gave English: nocuous (harmful — rare but the base of the more common innocuous, "not harmful"), innocent (in-nocēns, "not harming" — the innocent are etymologically those who do no harm), nuisance (from Old French nuisance, from Latin nocēre — a nuisance is that which harms), and noxious (harmful, poisonous).
Greek nékys (νέκυς, "corpse, dead body") and the related nekrós (νεκρός, "dead") may continue the root through the concept of the harmed/killed one. From nekrós English inherits: necropolis ("city of the dead" — a cemetery), necromancy (nekromanteía, "divination by consulting the dead"), necrosis (tissue death), and nectar (nek-tar, possibly "overcoming death" — the drink of the gods, the substance that conquers mortality, connecting this root to *mer- the death root and *terh₁- the crossing/overcoming root).
Sanskrit naśyati ("he perishes, he is destroyed") and the related nāśa- ("destruction, ruin") confirm the Indo-Iranian reflex.
The word's semantic range from hostility (nocēre) through corpses (nekrós) to innocence (innocēns) shows how a root for harm can generate both destructive and protective vocabulary: to be innocent is to be defined by the ABSENCE of the harm this root names.