Internecine fighting is fighting inside one group. A party tears itself apart. A family is at war with itself. A country is in a civil fight. The people hurting each other belong together, and that is the sting of the word. It is a formal, bookish adjective. News and history like it: internecine warfare, internecine struggles, internecine conflict. You almost never hear it in casual speech. It usually sits in front of a noun about fighting; people rarely say “the struggle was internecine” in everyday talk. Though actually, the Latin word it comes from meant “deadly, murderous” — a massacre — and the inter- piece there did not mean “between”. In Latin it was more like a strong extra, or “down, under”, the way it works in some other Latin words about death. English speakers later read it through English inter- (“between”), and that is how we got today’s “inside the same group” meaning. So the tidy classroom story “inter- = between, so between members” is how English uses it now, but it is not how the Latin was built. Some older or very careful writers still let it mean “mutually destructive” even when the two sides are not one family. Most of the time now, if you see it, someone is talking about a fight in the house.

內鬥的；自相殘殺的（內部互打，不是國與國）