slapper (n.)

late 15c., slappen, "strike or smack with the open hand," from slap (n.). By 1836 as "to put into place." As an adverb, 1670s, "suddenly;" 1829, "directly." Related: Slapped; slapping.

1767, "uncommonly large thing," originally and especially an audacious lie, formed as if from whop (v.) "to beat, overcome." Whopping "large, big, impressive" is attested by 1620s.

1794, "uncommonly large thing," agent noun from smash (v.); compare whopper, slapper, etc. By 1822 as "machine which crushes;" by 1829 as "a heavy blow."