Internecine comes from the Latin internecinus ("fought to the death" or "destructive"), which traces to the verb "necare" ("to kill") and the prefix inter-. ("Inter-" usually means "between" or "mutual" in Latin, but it can also indicate the completion of an action.) Internecine meant "deadly" when it appeared in English in the early 17th century, but when Samuel Johnson entered it in his dictionary almost a century later, he was apparently misled by "inter-" and defined the word as "endeavouring mutual destruction." Johnson's definition was carried into later dictionaries, and before long his sense was the dominant meaning of the word. "Internecine" developed the association with internal group conflict in the 20th century, and that's the most common sense today.