noun

• (chiefly UK, Australia) Aggressive behaviour; loud, intimidating behaviour that convincingly threatens violence without necessarily actually becoming violent.
  "Move along, lads; we don't want any aggro."
• (gaming) A measure of how belligerent a player is – a high value may inspire either avoidance or preemptive hostile action from enemies.
  "To reduce one player's aggro level, the other must build up his own."
• (gaming) Hostile attention from an enemy that should target players with better defenses.
  "You just pulled aggro off the tank and wiped the raid."

adj

• (Australia, New Zealand, slang) Angry.
  "“The Chuckster say you two be finding aggro idiot who kilt Long,” said Dude. I thought I understood but looked at Charles for help. “Aggro means pissed off.” translated Charles. He still hadn't cracked a smile. Tardiness made Charles aggro."
• (gaming) Liable to attack without being attacked first (said of monsters).
• (TCGs, of a deck) Designed for aggressive play that seeks to defeat the opponent in the early stages of the game. (of a player) Favoring such decks.
• (British, US, slang) Hardcore, aggressive.
  "At the beginning of the twenty-first century the physically adept aggro femme had become a recurring motif in new circus and physical theatre."

verb

• (gaming, intransitive) To become aggressive towards the player's character.
  "They'll aggro on you if you get too close."
• (gaming, transitive) To cause (a non-player character) to become aggressive towards the player's character.
  "I aggroed the dragon by stealing from its hoard, and it killed me."