[Harimwakairi]

It's technically a type of cheating, but it's used here to mean profoundly bad luck.

A cold deck is one that has been rigged to the advantage of one player, and then swapped in after the cut and shuffle (hence its "coldness"). Typically the deck would be stacked to give the victim a fantastically good hand, but the cheater one that's slightly better. The victim would bet a great deal thinking their hand was good only to lose at the end.

Players now use the term to describe the rare occasions when someone loses with a particularly strong hand in a fair game.

[Phillipharryt]

Regarding your last paragraph, that would be a bad beat. A cold deck is a string of poor cards, perhaps an entire sitting without getting any strong hands.

[jordonias]

You're not wrong about it being a bad beat, but I think he was thinking of a "cooler".

[ddingus]

I think so too.