Chool it is a portmanteau of "chill out" and "cool it". Used when someone is either getting angry at nothing or being a fuckwad. "Chool out" is an alternative form of the phrase.
Guy 1: I'M SO ANGRY RIGHT NOW!
Guy 2: Dude, just chool it!
or
Guy 1: Wow, John is such a fuckwad.
Guy 2: I know, he needs to chool it.
1) Texas Hold'Em - burning a card before the flop, especially when pre-flop betting gets huge. Refers to pulling the pin on a grenade, but not throwing it immediately so it explodes a shorter time after throwing it.
2) To start a fight
1) Three guys went all-in, but we had to scrap the hand because Frank zoned out and cooked it before all the betting was done.
2) Some fool mooned Gary, so he chose to cook it right there in front of the bar.
It's a dynamic phrase that could be used to say someone's attractive, or if food looks good. It's antonym is "cook it down" which could mean that someone's nasty, shouldn't sport that type of clothing, or if the food isn't as good as you thought.