Suckout is a slang poker term for when someone draws out on their opponent.
Usage: For example – Let's say you have pocket Aces, and your opponent pocket Kings. You both go all in. The cards come down, and your opponent catches a King to give him the winning hand. Your opponent has sucked out on you. Ex – "Yeah, I was ahead when the money went in, and then he sucked out on me on the turn when he caught his spade."