Short for aggressive or aggression. Can be used to describe how a person is behaving. Also in MMORPGs such as World of Warcraft it refers to an enemy attacking you if you get too close to it. In MMORPGs certain enemies have different ranges that will trigger them to attack. Some enemies can attack even when you are fairly far away and some only attack when your characters is right next to the enemy or in very close proximity. Higher level enemies tend to have a larger range than lower level ones.
"Chill out, what is with the aggro today?"

"That idiot in our group just pulled aggro on a group of elite murlocs!"

"You're pulling fucking aggro from them! Run for it!"
by IceWarm January 25, 2006
In an MMORPG when you "piss off" an NPC(non-player character) and it enters a combat state with your character. Most often used when multiple NPC's are attacking you because you got too close or you attacked them.
"I just aggro'd 15 level 80's! I need a heal!"
by lowell August 9, 2005

1 aggressive behaviour usually in the context of football related disorder.

2 to experience difficulties

abbreviation of aggravation
1 John: "How did you get on at Spurs on Saturday?"

Dave: "We beat them again and had all the usual aggro with the 'Yids' outside."

2 John: "Why don't we just get the bus to Spurs from Finsbury Park"

Dave: "Because every time we do that we get marched down Seven Sisters Road like cunts and get loads of shit off the old bill. Fuck that, I don't need all that aggro."
by sooner_gooner July 11, 2009
a term that has been used for decades by surfers and other water-headed types to describe someone who is overly and unesscessarily agressive, competitive, or assinine whilst in the water. prolonged exposure to the aggro can ruin even the best session, so "deal" with them immediately (read: choke with your leash then return to your state of zen-like calm once they are dead)
Watch out for those SoCal types, theyre aggro, they should stay in Cali where they fucking belong.
by East End Hui January 25, 2005
Noun: Aggro/agro
Verb: Aggroing
Adjective: Aggroed

A term used by many gamers referring to tipping off any NPC or animal, and causing it to want to fight you.
Example from Ark: Survival Evolved

Holy crap I just aggroed the Tickle Chicken!!! Run!!
by Gamer4200 December 19, 2019
In MMORPG's and MUD's, means that an NPC will attack pre-emptively, instead of only returning an attack.
"That new monster they put in the newbie area is aggro."
by Kenthar July 6, 2004