Green’s Dictionary of Slang

a hard, resounding slap or blow.

1772		Bridges Burlesque Homer (3rd edn) 491: But ajax gave him two such spankers, / They smarted worse than nodes and shankers.
1840		T. Haliburton Letter-bag of the Great Western (1873) 210: To mock him, they said, as he was a ‘stern’ man, they would treat him to a ‘spanker,’ and cut him with lashings dreadfully.