My old school friend, the eminent prosthodontist Dr. Simon Gamer (of Canadian birth), brought up a useful word in a recent telephone conversation, viz. internecine, which Merriam-Webster glosses as follows:
[¦intər¦ne|ˌsēn, -nē|, |ˌsīn, |sən, |sə̇n; ¦intərnə̇¦sēn; ə̇n‧ˈtərnəˌsēn, -nəsə̇n, -nəˌsīn]
1 a : marked by great slaughter : deadly <the alternatives only of internecine war or absolute surrender — W. E. Gladstone> b : involving or accompanied by mutual slaughter : mutually destructive <zealots who stabbed each other in internecine massacre — F. W. Farrar>
2: of, relating to, or involving conflict within a group; broadly : internal <absorbed in incurable, rancorous internecine feuds — Barbara Ward> <a bitter internecine struggle among artists — Roger Fry>
Origin of INTERNECINE
Latin internecinus, from internecare to destroy, kill (from inter- + necare to kill, from nec-, nex violent death) + -inus -ine — more at noxious
First Known Use: 1663 (sense 1a)
Note the swarm of extant pronunciations, betokening a confusion among speakers, who are typically unused to hearing the word spoken (as is so often the case with bookish words in contemporary American English). The preferred pronunciation (nota bene) is [in(t)ərˈnēsīn].