WordNet (r) 3.0 (2006):

flump
    v 1: fall heavily [syn: flump, flump down]
    2: set (something or oneself) down with or as if with a noise;
       "He planked the money on the table"; "He planked himself into
       the sofa" [syn: plank, flump, plonk, plop, plunk,
       plump down, plunk down, plump]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "flump":
   bump, cave, cave in, chink, click, clink, clop, clump, clunk,
   crump, decline, droop, dull thud, flick, flop, flop down,
   flump down, founder, give way, go down, lapse, lower, pad, pat,
   patter, pitapat, pitter-patter, plop, plop down, plump, plunk, pop,
   rap, sag, set, settle, settle down, sink, sink down, slouch, slump,
   slump down, submerge, subside, swag, tap, thud, thump, tick,
   tinkle, tunk