The Rice University Neologisms Database

Refers to when a non-player character(s) attacks a player character within the non-player character’s range of action before the player character has engaged in or initiated combat. The term appears to be a shortening of aggression that has been creatively misspelled, or the shortening may have entered from French, as their spelling has no second g. ‘Agro’ is only commonly used among the gaming subset populations, and may have alternatively come about as a clipping used in video game manuals to describe the properties of different non-player characters and their combat techniques.