Source : Webster's Revised Unabridged Dictionary (1913)

Overwhelm \O`ver*whelm"\, v. t. [imp. & p. p. {Overwhelmed}; p.
   pr. & vb. n. {Overwhelming}.]
   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.

            The sea overwhelmed their enemies.    --Ps. lxxviii.
                                                  53.

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

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

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

   2. To project or impend over threateningly.

            His louering brows o'erwhelming his fair sight.
                                                  --Shak.

   3. To cause to surround, to cover. --Papin.

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

Source : WordNet®

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}]