Definition of PAT
In some variations of poker, such as 2-7 Triple Draw, it is possible to discard cards to get more cards. Discarding 3 cards rather than one for example will tell our opponent that our starting game was not very good, otherwise we would have kept our cards. If we decide not to change any of our cards, then we will say that we are PAT. The expression could be translated as "being ready". PAT is the fact that we don't change any of our cards in our hands. If you play 2-7 Triple Draw online, you will see the number of cards that each day has changed after each round, and if a player has not changed any of them, you will see the expression PAT written on that player.
PAT in Chess
Here, no connection with poker, but as you will often hear the expression PAT associated with the world of chess, you might as well explain what it is exactly about. In chess, we talk about PAT when a player is not in check, but every possible move he can make puts him in check. In chess, a player is not allowed to make a move that puts him in check. The PAT often comes at the end of the game when a player has only his king.