A pat hand is one that is complete as it stands, so the player takes no more cards. In blackjack, the word describes a total of 17 to 21 that has not busted; drawing again would put the hand at serious risk, so the usual play is to stand. In draw poker, standing pat means keeping all five cards on the draw because the hand is already strong, such as a straight, flush or full house.
The expression has moved into everyday English as “stand pat”, meaning to stick with a decision. For operators, pat hands matter mainly in game design and analytics: blackjack strategy charts, dealer rules such as standing on soft 17 and video poker hold logic all rest on knowing when a hand needs no further cards. See also standing hand.