infight

The story

1. Old English · derived frominfiht
2. Middle English · derived frominfighten“to attack”
3. English todayinfight

Apparently a back-formation from infighting, perhaps continuing Middle English infighten (“to attack”), from Old English infiht, infeoht (“an attack made upon a person by another in the same house or between members of a household”), equivalent to in- + fight. Compare Middle English infiht (“an internal fight, a brawl within a household”).