Definition of PAT
In some poker games, such as 2-7 Triple Draw, it is possible to throw away cards to get more cards. Throwing away three cards instead of one, for example, will tell our opponent that our original play was not very good, otherwise we would have kept our cards. If we decide not to change any of our cards, we are said to be PAT. This could be translated as "being ready". PAT is the act of not changing any of our cards in our hands. If you play 2-7 Tiple Draw online, you'll see the number of cards each day has changed after each round and if a player hasn't changed any, you'll see the phrase PAT written on that player.
PAT in chess
Here, no link with poker, but as you will often hear the expression PAT associated with the world of chess, we might as well explain what it is exactly. In chess, PAT is 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.