WordNet (r) 3.0 (2006):

wham
    v 1: hit hard; "The teacher whacked the boy" [syn: whack,
         wham, whop, wallop]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "wham":
   bang, bash, bat, belt, biff, blast, bonk, boom, bump, burst, bust,
   clap, clash, clip, clobber, clout, clump, coldcock, crack, crash,
   crump, cut, dash, deal, deal a blow, deck, fetch, fetch a blow,
   flap, flop, hit, hit a clip, jab, knock, knock cold, knock down,
   knock out, let have it, paste, plunk, poke, punch, rap, report,
   slam, slap, slat, slog, slug, smack, smash, smite, snap, soak,
   sock, splat, strike, strike at, swap, swat, swipe, tap, thump,
   thwack, wallop, whack, whap, whomp, whop, yerk