spanker

nouns

a hitter who slaps (usually another person) with an open hand

"someone slapped me on the back and I turned to see who the slapper was"; "my father was the designated spanker in our family"

slapper, spanker

is derived from

a fore-and-aft sail set on the aftermost lower mast (usually the mizzenmast) of a vessel

spanker

is a type of (narrower)

any sail not set on a yard and whose normal position is in a fore-and-aft direction