Merriam-Webster's Collegiate® Dictionary, 11th Edition
SLAP
I. noun
Etymology: Middle English slop, from Middle Dutch; akin to Middle Dutch slippen to slip
Date: 14th century
opening , breach ,
II. transitive verb (slapped; slapping)
Etymology: akin to Low German slapp, noun blow
Date: 15th century
1. to strike sharply with or as if with the open hand, to cause to strike with a motion or sound like that of a blow with the open hand, to put, place, or throw with careless haste or force , to assail verb ally ; insult , to subject to a penalty,
III. noun
Date: 1606
1. a quick sharp blow, a blow with the open hand, a noise like that of a slap, rebuff , insult ,
IV. adverb
Etymology: probably from Low German slapp, from slapp, noun
Date: 1672
directly , smack