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]