The Collaborative International Dictionary of English v.0.48:

Bombshell \Bomb"shell`\, n.
   1. A bomb. See Bomb, n.
      [1913 Webster]

   2. something or someone that stuns or amazes, especially
      suddenly and unexpectedly; as, the news of the president's
      affair was a political bombshell; a blonde bombshell.
      [PJC]

WordNet (r) 3.0 (2006):

bombshell
    n 1: an entertainer who has a sensational effect; "she was a
         blonde bombshell"
    2: a shocking surprise; "news of the attack came like a
       bombshell" [syn: bombshell, thunderbolt, thunderclap]
    3: an explosive bomb or artillery shell

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "bombshell":
   aerial bomb, antipersonnel bomb, astonishment, blockbuster, blow,
   bomb, carcass, catch, concussion grenade, depth bomb, depth charge,
   earthshaker, eye-opener, fire bomb, gas grenade, grenade,
   hand grenade, incendiary bomb, incendiary grenade,
   infernal machine, joker, kicker, peripeteia, petard, revelation,
   shock, shocker, staggerer, startler, surprisal, surprise,
   surprise ending, surprise package, surprise party, switch,
   tear-gas grenade, thunderbolt, thunderclap, time bomb,
   wall grenade