Lewis Short
(adjective) : internĕcīnus or internĕcīvus, a, um, internecio
* Deadly, murderous, destructive (class.): bellum,Cic. Phil. 14, 3, 7; Liv. 9, 25 fin.; 22, 58, 3: odia,Just. 6, 6: internecini actio, for poisoning, Cod. Th. 9, 1, 14: internecini judicium, of one who has committed perjury, acc. to Isid. Orig. 5, 26. — Adv.: internĕcīnē, with utter destruction: cuncta disperdere,Amm. 27, 9, 6.
Charlton T. Lewis, Charles Short, A Latin Dictionary