"Aggro" is short for aggravate. Anyone who says it is short for the word aggressive need only test their misinformed theory by replacing the word "aggro" with "aggressive" and then with "aggravate/aggravation/aggravated". In short, aggressive is a state. An enemy is aggressive towards all players, but who they attack is determined by their level of aggravation towards the different members of the player group, and is affected by DPS, abilities that have higher aggro than they otherwise would based on the amount of damage that ability indlicts, and healers who do 0 damage (and thus are not aggressive towards the enemies at all, yet still cause the enemy aggravation. Here are some sample sentences.

"Don't aggro the boss until you clear the adds." Which makes sense in the English language in place of aggro, aggravate or aggressive? Another example, "Those enemies have a very large or small aggro radius." Less clear, but still, aggravation works better than aggressive or aggression, because mobs that trigger based on range are considered "aggressive" already, and those that only trigger on damage taken are "neutral" and those that will not retaliate at all are "passive" and the difference in their nature usually made clear by the game using universally understood color systems. (Red, yellow, green; red, green, blue; etc.)

When aggro is used to mean "threat" as in, "Idiot X DPS class out aggro'd the tank, that's what caused the wipe."; it again makes more sense that it means "X DPS caused the enemy more aggravation than the tank, thus causing the enemy to shift it's focus." A few more examples that use aggro to mean "aggravation": "DPS should allow the tank a few moments to build aggro before starting their rotations, otherwise they won't have a chance to hold aggro."

Also, my personal favorite "This tank sucks so bad the healer pulls aggro." (Meaning the tank is so poorly geared that they are taking so much damage that the healer has to use tons if heals, which create threat the same way as DPS does, causing the monster to become more aggravated with them than the tank, or the tank doesn't know how to play their class and use the tools provided to artificially inflate their aggro above that of other roles.

"Aggro" is short for aggravate, instead of aggressive.

In short, aggressive is a state. An enemy is aggressive towards all players, but who they attack is determined by their level of aggravation towards the different members of the player group. Depending on the game, it can be affected by a number of factors such as DPS, abilities, or even healing allies.

Here are some sample sentences:

• "Don't aggro the boss until you clear the adds."
  • Aggravate makes more sense than aggressive in this context.
• "Those enemies have a very large or small aggro radius."
  • Aggravation works better than aggressive or aggression, because mobs that trigger based on range are considered "aggressive" already.
• "X DPS class out aggro'd the tank, that's what caused the wipe."
  • It makes more sense that it means "X DPS caused the enemy more aggravation than the tank, thus causing the enemy to shift its focus."