The Collaborative International Dictionary of English v.0.48:

Slap \Slap\, n. [OE. slappe; akin to LG. slappe, G. schlappe;
   probably of imitative origin.]
   A blow, esp. one given with the open hand, or with something
   broad.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slap \Slap\, v. t. [imp. & p. p. Slapped; p. pr. & vb. n.
   Slapping.]
   To strike with the open hand, or with something broad.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slap \Slap\, adv. [Cf. LG. slap, G. schlapp. See Slap, n.]
   With a sudden and violent blow; hence, quickly; instantly;
   directly. [Colloq.] "The railroad cars drive slap into the
   city." --Thackeray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

slap
    adv 1: directly; "he ran bang into the pole"; "ran slap into
           her" [syn: bang, slap, slapdash, smack, bolt]
    n 1: a blow from a flat object (as an open hand) [syn: slap,
         smack]
    2: the act of smacking something; a blow delivered with an open
       hand [syn: smack, smacking, slap]
    v 1: hit with something flat, like a paddle or the open hand;
         "The impatient teacher slapped the student"; "a gunshot
         slapped him on the forehead"

Moby Thesaurus II by Grady Ward, 1.0:

160 Moby Thesaurus words for "slap":
   Parthian shot, abruptly, altogether, attack, back answer, bang,
   bash, bat, beat, biff, blip, blow, box, box the ears, buffet, bump,
   burst, bust, cast, castigate, caustic remark, censure, chop, chuck,
   clap, clash, clear, clout, comeback, completely, contumely, crack,
   crash, criticism, crump, cuff, cut, cutting remark, dash, despite,
   dig, ding, directly, drub, dump, entirely, exactly, flap, flay,
   fleer, fling, flop, flout, foolery, fully, gibe, gibing retort, go,
   hastily, haymaker, hit, hurl, impetuously, impulsively, indignity,
   insult, jab, jape, jeer, jest, knock, lash, leg-pull, like a flash,
   like a thunderbolt, mock, of a sudden, on short notice,
   parting shot, perfectly, plank, plop, plumb, plump, plunk,
   point-blank, poke, pop, precipitantly, precipitately,
   precipitously, precisely, punch, put-down, put-on, quip, quite,
   rap, rebuff, rebuke, report, reprimand, reproof, right,
   rude reproach, scathe, scoff, score, scourge, scurrility, sharp,
   short answer, shot, slam, slap on, slap the face, slash, slat,
   sling, smack, sock, spank, splash, splat, squarely, stab, stagger,
   startlingly, straight, strike, stripe, sudden, suddenly,
   surprisingly, swap, swat, swipe, tap, taunt, throw, thrust, thwack,
   token punishment, toss, try, twit, unawares, unexpectedly, utterly,
   verbal thrust, wallop, whack, wham, whap, whip, whirl, whomp, whop,
   without notice, without warning