Acting generally retarded, socially incompetent, and/or in an idiotic manner. Also, if you are being a max you most likely have either a small penis or a huge clitoris. You can also be accused as "Being A Max" if you cum apple juice. In all, if you are told that you are "Being A Max" you might consider crying and yelling "Shut the Fuck Up" in an annoying tone. It is not a compliment if you are called a max. (A.K.A. Max-A-Faggot)
If your friend is shooting a lacrosse goal when he/she is 1.7 feet away, and misses on an open goal you can say, "Dude! I could have made that goal stop being a Max!"
In a skateboarding game called SKATE, for one of his tricks he might board slide the end of a rail (Which is illegal in the game) and for his next trick he might board slide MOST of the rail (Also illegal) you can tell him/her to "Stop being a Max! PLEASE"