Source : Webster's Revised Unabridged Dictionary (1913)

Thunderbolt \Thun"der*bolt`\, n.
   1. A shaft of lightning; a brilliant stream of electricity
      passing from one part of the heavens to another, or from
      the clouds to the earth.

   2. Something resembling lightning in suddenness and
      effectiveness.

            The Scipios' worth, those thunderbolts of war.
                                                  --Dryden.

   3. Vehement threatening or censure; especially,
      ecclesiastical denunciation; fulmination.

            He severely threatens such with the thunderbolt of
            excommunication.                      --Hakewill.

   4. (Paleon.) A belemnite, or thunderstone.

   {Thunderbolt beetle} (Zo["o]l.), a long-horned beetle
      ({Arhopalus fulminans}) whose larva bores in the trunk of
      oak and chestnut trees. It is brownish and bluish-black,
      with W-shaped whitish or silvery markings on the elytra.

Source : WordNet®

thunderbolt
     n 1: a discharge of lightning accompanied by thunder [syn: {bolt},
           {bolt of lightning}]
     2: a shocking surprise; "news of the attack came like a
        bombshell" [syn: {bombshell}, {thunderclap}]