·noun A blow with the open hand; a slap.
II. Spank ·vt To strike, as the breech, with the open hand; to Slap.
III. Spank ·vi To move with a quick, lively step between a trot and gallop; to move quickly.
To run neatly along, beteeen a trot and gallop. The tits spanked it to town; the horses went merrily...
Dictionary of The Vulgar Tongue by Francis Grose