Green’s Dictionary of Slang

1. a slap with an open hand [later use is SE].

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Spanks, [...] blows with the open hand.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1857		Bell’s Life in Sydney 17 Oct. 3/3: Maria [...] popped in a heavy spank on the dexter dimple which brought tho pink ribbons to the ground.
1866		Wild Boys of London I 7/1: He hits him a spank in the jaw.

2. the breaking of a shop window before grabbing whatever can be reached.

1812		Vaux Vocab. of the Flash Lang.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1847		Bell’s Life in Sydney 10 Apr. 2/3: Ike sent in a left-handed spank on the winker.

3. a punch.

1853		N.Y. Clipper 13 Aug. 1/7: A spank on the gob from Barry’s left.
1855		(con. 1824) Fights for the Championship 95: Ward floored him with a heavy spank on the throttle.
1897		Mirror of Life 27 Jan. 3/2: Sam Millard sprang up and caught Joe Wareham a heavy spank on the nose.
1955		B. Hill Boss of Britain’s Underworld 203: ‘All right George [...] Give him a spank.’ George walked over to the dago and slung a right-hander on his chin.