The Collaborative International Dictionary of English v.0.48:

Overwhelming \O`ver*whelm"ing\, a.
   Overpowering; irresistible. -- O`ver*whelm"ing*ly, adv.
   [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):

overwhelming
    adj 1: so strong as to be irresistible; "an overpowering need
           for solitude"; "the temptation to despair may become
           overwhelming"; "an overwhelming majority" [syn:
           overpowering, overwhelming]
    2: very intense; "politics is his consuming passion";
       "overwhelming joy" [syn: consuming, overwhelming]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "overwhelming":
   absolute, adducible, admissible, agitating, amazing, astonishing,
   astounding, attestative, attestive, authentic, awe-inspiring,
   awesome, based on, bewildering, breathtaking, burdensome, certain,
   charged, circumstantial, cliff-hanging, conclusive, confounding,
   convincing, crushing, cumulative, damning, decisive, determinative,
   devastating, disquieting, distracting, disturbing, documentary,
   documented, electric, evidential, evidentiary, ex parte, exciting,
   exhilarating, eye-opening, eye-witness, factual, final, firsthand,
   formidable, founded on, galvanic, grounded on, heady, hearsay,
   heart-expanding, heart-stirring, heart-swelling, heart-thrilling,
   impenetrable, implicit, impregnable, impressive, incontestable,
   incontrovertible, indicative, indisputable, indomitable,
   inexpugnable, inflammatory, insuperable, intoxicating, invincible,
   inviolable, invulnerable, irrefutable, irresistible, jarring,
   jolting, maddening, material, mind-blowing, mind-boggling,
   monumental, moving, nuncupative, overcoming, overmastering,
   overmatching, overpowering, perturbing, piquant, presumptive,
   probative, prodigious, provocative, provoking, ravishing, reliable,
   resistless, significant, soul-stirring, spectacular,
   spirit-stirring, staggering, startling, stimulating, stimulative,
   stirring, striking, stunning, stupefying, suggestive, sure,
   surprising, suspenseful, suspensive, symptomatic, tantalizing,
   telling, thrilling, thrilly, troubling, unassailable, unbearable,
   unbeatable, unconquerable, uncontrollable, unendurable, unsettling,
   unsubduable, unsurmountable, unyielding, upsetting, valid,
   weighty