1660s, "deadly, destructive," from Latin internecinus "very deadly, murderous, destructive," from internecare "kill or destroy," from inter (see inter-) + necare "kill" (see noxious). Considered in the OED as misinterpreted in Johnson's Dictionary [1755], which defined it as "endeavouring mutual destruction," on association of inter- with "mutual" when the prefix supposedly is used in this case as an intensive. From Johnson, wrongly or not, has come the main modern definition of "mutually destructive."