cooler = cold deck. they both mean you have a basically unfoldable hand (given stack size) against a better hand:

example, KK vs AA, set over set, K high flush vs A high flush, 9x on a 87654 board if opp has 9T, etc.

classic example from HSP was the pot with Daniel and Gus where they both flopped sets - cooler for Gus- until Gus made quads (super cooler!).