Etymology
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internecine (adj.)
1660s, "deadly, destructive," from Latin internecinus "very deadly, murderous, destructive," from internecare "kill or destroy," from inter (see inter-) + necare "kill" (from PIE root *nek- (1) "death").

Considered by OED as misinterpreted in Johnson's Dictionary [1755], which defined it as "endeavouring mutual destruction," but a notion of "mutually destructive" has been imported into the word in English because in English inter- usually conveys the idea of "mutual." The Latin prefix is said to have had here only an intensive sense; "the Latin word meant merely of or to extermination ... without implying that of both parties" [Fowler].
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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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