slap (n.) Look up slap at Dictionary.com
mid-15c., probably of imitative origin, similar to Low German slappe, German Schlappe. Figurative meaning "insult, reprimand" is attested from 1736. Slap-happy (1936) originally meant "punch-drunk." Slap on the wrist "very mild punishment" dates from 1914.
slap (v.) Look up slap at Dictionary.com
late 15c., "strike with the open hand," from slap (n.). As an adverb, 1670s, "suddenly;" 1829, "directly." Related: Slapped; slapping.