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].