in·ter·ne·cine/ˌɪn tərˈni sin, -saɪn, -ˈnɛs in, -ˈnɛs aɪn/

From dictionary.com: Adjective 1) of or pertaining to conflict or struggle within a group: an internecine feud among proxy holders. 2) mutually destructive. 3) characterized by great slaughter; deadly.

When I read this it was used in the context of a conflict within a group, but I much prefer “great slaughter”: so much more dramatic. Now I’ll think of it that way every time I read it.

Etymology

From etymonline.com: 1660s, “deadly, destructive,” from L. 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.”

Feature image: Arista romano, battaglia di zama by Cornelis Cort.