About as cool as a person can possibly be, sort of like James Dean or someone like that, but even cooler. Possibly originated from SportsCenter. Has nothing to do with temperature.
I was as cool as the other side of the pillow back in high school. I used to sit in the back of the class in my leather Harley Davidson jacket and sunglasses, smoking my Marlboro Reds and drinking whiskey out of a silver flask. I'd just sit back there, all laid back across the seat, with my arms around the two hottest girls in the school. The teacher would tell me to put my cigarette out and stop drinking, and I would blow smoke in her face and say, "Make me." But she wouldn't make me because I was just too cool.
No, not really, actually I was a fucking dork who watched Star Trek marathons and still haven't lost my virginity except for my dog that one time.
No, just kidding again, I was pretty normal, but I always used to look at the other side of the pillow and wished that someday I could be that cool. Sadly, it never happened.