Usually a guy who is good at sports, drinking, partying, and pleasing the opposite gender. These guys are a special breed of guy that can only be found in the 951. There is a group of friends who coined themselves as THEE Sick Ass Dudes
person 1: "that guy is badass!"
person 2: "yeah, he's a sick ass dude"
person 1: "oh for reals? No wonder!"
More often than not, it is used in a derogatory manner. Can also be a term of endearment. Personally I mostly say it when someone tells me they did something I have no interest in.