A sap is a person who stands behind a player and uses psychological tricks to encourage that player to make more reckless bets in order to increase the chances of winning.
Blackjack is a casino game in which the player bets on whether the dealer will either hit a hand or not. If the player is correct, they win money based on their bet; if they are incorrect, they lose their bet.
The difference between a sap and a blackjack player is that a sap tries to get the player to make more risky bets, while a blackjack player only tries to avoid losing money.