First usage among gaming communities in the late 1990s; derived from running after an opponent while facing their rear (ass).
assrun (third-person singular simple present assruns, present participle assrunning, simple past and past participle assran)
- (US, video games, slang) To follow and shoot at (somebody in a first-person shooter game) after having passed them.
- One of your players assran me at every opportunity.
- You were assrunning for the whole game.