because they got very drunk one day after winning a big case. It's an avocado, luckily it's small and actually looks like an avocado. Matt's pissed, in part because now he has a mark that people can identify him as, and pissed at Foggy, whose job it is as the sighted part of the pair to keep him from doing dumb shit like this.
Gen: Matt and Foggy have matching tattoos