Aggravation; bother.

Aggressive behaviour; loud, intimidating behaviour that convincingly threatens violence without necessarily actually becoming violent.

"Move along, lads; we don't want any aggro."

A measure of how belligerent a player is – a high value may inspire either avoidance or preemptive hostile action from enemies.

(MMORPG slang) Hostile attention from an enemy that should target players with better defenses.

(MMORPG slang) To become aggressive towards the player's character.

"Do they aggro on sight or sound?"

Angry.

(MMORPG slang) Liable to attack without being attacked first (said of monsters).

Hardcore, aggressive.