An ARG is an online game with stories and characters whom you must assist in some manner via solving puzzles/challenges. Bigger ARGs may have real-life challenges, such as a meeting with one of the characters or picking up information somewhere.
A few well-known ARGs are I Love Bees, Seen James?, The Art of the Hiest and The Beast.
May be applied to humans and other bipeds in a fashion similar to cankles.
That bloke should quit eating so much cheese, he's gonna blow an arg.