[1] : The Collaborative International Dictionary of English v.0.48

overwhelm \o`verwhelm"\, 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
see also:
overwhelmed overwhelming 

[2] : The Collaborative International Dictionary of English v.0.48

overwhelm \o"verwhelm`\, n.
   the act of overwhelming. r.
   1913 webster

[3] : WordNet (r) 2.0

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

[4] : 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

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Overwhelm, definition of term: Overwhelm
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