A hitter who slaps (usually another person) with an open hand:
— My father was the designated spanker in our family.

synonym: slapper.

Roget 192: size, magnitude, dimension, bulk, volume; largeness etc. adj.; greatness etc. (of quantity) 31; expanse etc. (space) 180; amplitude, mass; proportions.    capacity, tonnage, tunnage; cordage; caliber, scantling.    turgidity etc. (expansion) 194; corpulence, obesity; plumpness etc. adj.; embonpoint, corporation, flesh and blood, lustihood.    hugeness etc. adj.; enormity, immensity, monstrosity.    giant, Brobdingnagian, Antaeus, Goliath, Gog and Magog, Gargantua, monster, mammoth, Cyclops; cachalot, whale, porpoise, behemoth, leviathan, elephant, hippopotamus; colossus; tun, cord, lump, bulk, block, loaf, mass, swad, clod, nugget, bushel, thumper, whooper, spanker, strapper; Triton among the minnows" [Coriolanus]. mountain, mound; heap etc. (assemblage) 72.    largest portion etc. 50; full size, life size.    ... show more

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

Dutch: spanker