The Collaborative International Dictionary of English v.0.48:

Overwhelm \O"ver*whelm`\, n.
   The act of overwhelming. [R.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Overwhelm \O`ver*whelm"\, v. t. [imp. & p. p. Overwhelmed; p.
   pr. & vb. n. Overwhelming.]
   [1913 Webster]
   1. To cover over completely, as by a great wave; to overflow
      and bury beneath; to ingulf; hence, figuratively, to
      immerse and bear down; to overpower; to crush; to bury; to
      oppress, etc., overpoweringly.
      [1913 Webster]

            The sea overwhelmed their enemies.    --Ps. lxxviii.
                                                  53.
      [1913 Webster]

            Fearfulness and trembling are come upon me, and
            horror hath overwhelmed me.           --Ps. lv. 5.
      [1913 Webster]

            Foul deeds will rise,
            Though all the earth o'erwhelm them.  --Shak.
      [1913 Webster]

            Gaza yet stands; but all her sons are fallen,
            All in a moment overwhelmed and fallen. --Milton.
      [1913 Webster]

   2. To project or impend over threateningly.
      [1913 Webster]

            His louering brows o'erwhelming his fair sight.
                                                  --Shak.
      [1913 Webster]

   3. To cause to surround, to cover. --Papin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

overwhelm
    v 1: overcome, as with emotions or perceptual stimuli [syn:
         overwhelm, overpower, sweep over, whelm,
         overcome, overtake]
    2: charge someone with too many tasks [syn: overwhelm,
       deluge, flood out]
    3: cover completely or make imperceptible; "I was drowned in
       work"; "The noise drowned out her speech" [syn: submerge,
       drown, overwhelm]
    4: overcome by superior force [syn: overpower, overmaster,
       overwhelm]

Moby Thesaurus II by Grady Ward, 1.0:

201 Moby Thesaurus words for "overwhelm":
   afflict, aggrieve, amaze, anguish, answer, answer conclusively,
   argue down, astonish, astound, awe, awestrike, baptize,
   be prodigal with, be strong, beat, beat down, bedaze, bedazzle,
   bend, bewilder, blank, board, boggle, bowl down, bowl over, break,
   break down, bring down, bring low, bring to tears, bring to terms,
   bulldoze, bury, cascade, cataract, clobber, confound, confuse,
   confute, conquer, contradict, controvert, cream, crush, cut up,
   daze, dazzle, defeat, defeat utterly, deluge, demolish, demoralize,
   deny, desolate, destroy, devastate, dip, discombobulate, dismiss,
   dispose of, disturb, douse, downgrade, draw tears, drown,
   drown out, drub, duck, dumbfound, dumbfounder, dunk, embitter,
   engulf, escalade, finish, flabbergast, flood, flood the market,
   floor, foray, grieve, humble, humiliate, immerge, immerse, inroad,
   inundate, invade, jam, knock over, lick, lower, make a raid,
   make an inroad, master, merge, nonplus, oppress, outroar, outshout,
   overbear, overbrim, overcome, overdose, overequip, overflow,
   overfurnish, overlavish, overmaster, overpower, overprovender,
   overprovide, overprovision, overrun, oversell, overstock,
   oversupply, overtax, overthrow, overturn, pack a punch, paralyze,
   parry, paste, perplex, petrify, plunge in water, pour out,
   pour over, prostrate, put to silence, quash, quell, raid, rebut,
   reduce, reduce to silence, refute, ruin, run over, scale,
   scale the walls, schmear, settle, shatter, shellac, shock,
   shout down, shut out, shut up, silence, sink, skunk, slop, slosh,
   smash all opposition, smear, smother, snow under, sorrow, souse,
   spill, spill out, spill over, squash, squelch, stagger, startle,
   steamroller, storm, strike dead, strike dumb, strike with wonder,
   stun, stupefy, subdue, submerge, submerse, subordinate, subvert,
   suppress, surprise, swamp, sweep, take aback, take by storm,
   thrash, torment, trample underfoot, tread underfoot, trim,
   tyrannize, undermine, unman, upset, weigh down, whelm, whitewash,
   whomp, whop, wreck