Meaning

adjective (of conflict) within a group or organization

adjective characterized by bloodshed and carnage for both sides

originBorrowed from Latin internecīnus (“deadly”), from internecium (“a massacre, bloodbath; an eradication”) + -īnus. In Latin, the sememe 'between' was here not expressed by the prefix, it instead either had a somewhat emphatic meaning or meant "down, under", comparable to its use in other Latin terms related to death: see interficiō and intereō. The English current sense is thus a reanalysis of the…

Etymology from Wiktionary, CC BY-SA 3.0