Green’s Dictionary of Slang

back-hander n.

[SE backhander, a blow from the back of the hand]

1. [mid–late 19C] a piece of negative gossip delivered ‘behind someone’s back’.

2. [mid–late 19C] a drink taken out of turn or an extra drink taken while the decanter circulates.

3. [late 19C+] (also backhand) a bribe; a payoff for services rendered.

4. a blow delivered with the back of the hand.