Definition of suckout
A suckout in poker happens when we are heavily favored to win the hand, but the card we didn't want to see shows up. For example, you are all in on the turn on 2-2-5-7 and you have AA. Your opponent calls your all in with KK. Here you have a 95% chance of winning the hand. If a K falls on the river, which will happen a little more than 4% of the time, you are said to have had a suckout.
How to deal with suckouts?
Suckouts are inevitable. If you play poker a lot, you'll get several a day. The important thing is to stay calm and keep playing your A game. You don't want the tilt to be more costly than the suckout. If you lose your cool, you will end up losing more money on spew than on your suckout. Some players are able to take it well and still play good poker, but others are not. It's important to know yourself well. If you're the type of player who loses your temper when you get a big suckout, take a break and come back to the tables a little later.