today, it means a hand where you should always lose a lot of money if you play well. opponent just had better cards than you could have expected, or you got most of the money in while you were way ahead.
it comes from an old cheating term for a deck secretly introduced into the game to make one player lose a lot of money to another. Since the new deck was not "warm" from play, it was a cold deck or cooler, see. The crudest form would be to deal KK to the sucker, and give him a set of kings on the flop. The cheater would get AA, with a 3rd Ace on the river.
Here's a link to an example of a more
artful cooler
but usually now when somebody says this, they're not alleging cheating, just a hand that would be impossible to not lose money on.