Etymology
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innocuous (adj.)

"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.

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*nek- (1)
Proto-Indo-European root meaning "death." It forms all or part of: innocent; innocuous; internecine; necro-; necropolis; necrosis; necromancy; nectar; nectarine; nociceptive; nocuous; noxious; nuisance; obnoxious; pernicious.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nasyati "disappears, perishes," Avestan nasyeiti "disappears," nasu- "corpse," Old Persian vi-nathayatiy "he injures;" Greek nekros "corpse;" Latin nex, genitive necis "violent death, murder" (as opposed to mors), nocere "to harm, hurt," noxius "harmful;" Greek nekus "dead" (adj.), nekros "dead body, corpse;" Old Irish ec, Breton ankou, Welsh angeu "death."
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