Via Twitter, @I_Am_Maylin_Now suggested that I look at aggro, and when I said it was a term with which I was not familiar, he directed me to a World of Warcraft (WoW) wiki site with this definition:
Aggro is a jargon word in WoW, probably originally derived from the English words “aggravation” or “aggression”, and used since at least the 1960s in British slang. In MMORPGs [Massively multiplayer online role-playing games], such as WoW, aggro denotes the aggressive interests of a monster/NPC. Some examples are “We’ve got aggro!” and “Go aggro that monster”.
The OED bears out this out (regarding the origin), citing a 1969 article in It magazine (“At the moment kids are split up into different subcultural groups which have been driven by the system into a permanent state of aggro with each other”) and Martin Amis’s 1973 novel The Rachel Papers: “It wasn’t day-to-day aggro, nor the drooped, guilty, somehow sexless disgruntlement I had seen overtake many relationship.”
The dictionary doesn’t recognize aggro as a verb, but does locate an adjectival use in Australia, as in “My New York paintings were all pretty aggro, with plenty of black” (Sunday Mail of Brisbane, 1985).
As my Twitter friend might have predicted, the term seems to have entered the U.S. through the world of gaming, with the OED citing Wired magazine in 1999: “A gaming device that brings skiing, snowboarding, and skateboarding into your house without thrashing the furniture… The 2-foot X Board lets vid kids stand and deliver aggro drops, extreme spins, and more.” But it’s spread out since then, with Time Out New York asking in 2008, “Are bike enthusiasts too aggro in defending their rights to NYC’s thoroughfares?” and Vanity Fair referring last year to “steroids, which bulk the muscles and ramp up the aggro.”