in-fighting (n.)

1816, in pugilism, the practice of getting at close quarters with an opponent; see in + fighting. Old English infiht (n.) meant "brawl within a house or between members of a household." Middle English had infight (v.) "to attack" (c. 1300); the modern verb infight "fight at close quarters" (1916) appears to be a back-formation from in-fighting. Related: In-fighter (1812).