In poker, a cooler is a case in which somebody plays a very strong hand that justifies placing the maximum bet, but is still beaten by an even stronger hand. Most coolers are a result of betting high on the second best hand and losing to the nuts. Please note that getting a cooler does not imply misplaying the hand. In fact, usually the second best hand is so strong that it’s virtually impossible to fold. As a result, coolers are more than likely to put the losing player on tilt.
A good example of a cooler in No Limit Texas Hold’em would be if one player decided to go all in with a king-high straight and lost to an opponent who managed to complete an ace-high straight. Keep in mind that determining whether a specific situation was a cooler or not is somewhat subjective, as there is no official definition of this term.
Preparing yourself to deal with coolers and bad luck in general is considered to be an important part of developing the right poker mindset. Therefore, poker books and message boards that focus on the psychological side of the game tend to discuss this subject in depth in order to help players cope with the losses, and understand that coolers are unavoidable even with proper play.