Often times, slang a sexual act that involves thrusting your fist into someone's anus. Often times, it can mean anything from being a whore, anal sex, sucking multiple cocks at once (ololol) and overall, being a fat assed otaku.
Person 1: Dude, I just gave my girlfriend a Maxxy!
Person 2: I feel sorry for her anus.
Person A: Dude, my girlfriend pulled a Maxxy.
Person B: Well, she was a whore anyways bro. Let's go eat some tacos.