A cold deck is a deck of cards which has been purposely and specifically ordered/sorted in order to deal a very specific set of poker hands to specific players, generally to the advantage of one of the players.
The term comes from the original idea that the deck had been ordered very carefully and set aside earlier, and has only been brought out to deal this one particular poker hand. Because the cards haven't been handled by the dealer for the past few hands, the cards are "cold" instead of "warm".
In colloquial poker slang, a cold deck is the common complaint when an unlikely set of hands (usually including the board cards, if any) has been dealt in such a way that a player who had every reason to think they had the best hand in actuality had the worst hand. Players in modern times say this in jest (or in a whine); it is not generally taken to mean the player really thinks the dealer switched decks before the hand to cause the result.
An example of a cold deck: a player being dealt Ace-King of hearts in hold 'em, another player being dealt pocket queens, and the flop coming 10-Jack-Queen of hearts. If the turn brings another Queen, the player with pocket queens has every reason to believe their quad queens are the best hand, but the player with the royal flush is actually the winner. The holder of the queens might very well complain that he was "cold decked" (note that the phrase can be used as a verb).
For a true-life example of a cold deck, see this YouTube video from Britain which shows con men conning poker professionals with a literal cold deck.
See also: Cooler