A bribe, a secret payment.
A glass of wine given out of turn, the bottle having been handed backwards.
A player who plays a backhand shot.
A shot played backhand, a backhand stroke.
A surfer who approaches a swell with the wave behind them ("on their backhand") rather than facing the wave ("on their forehand").
A blow with the back of the hand.
Money paid, especially at regular intervals, to criminals or to corrupt officials who threaten to cause harm to the payer or to his or her business if the money is not paid.