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.
A bribe.
In a transaction, something extra on the side.
A handout.
A tip or gratuity.