A cooler is a strong poker hand that loses to an even stronger hand in a situation that is unlikely to be avoided. In other words, it’s a situation in which a player didn’t make any mistakes but lost the hand to a superior holding.
For example, a cooler is when a player has a full house after the flop, but their opponent makes a four-of-a-kind at the same time. The player holding a full house just got “coolered,” and there is nothing he can do about it.
No one can fault a player for losing in such a situation, and you will be on the opposite side of the cooler and win the hand just as often in the long run.