Source : Webster's Revised Unabridged Dictionary (1913)

Bombproof \Bomb"proof`\, a.
   Secure against the explosive force of bombs. -- n. A
   structure which heavy shot and shell will not penetrate.

Source : WordNet®

bombproof
     adj : able to resist the explosive force of bombs and shells;
           "bombproof shelter" [syn: {shellproof}]
     n : a chamber (often underground) reinforced against bombing and
         provided with food and living facilities; used during air
         raids [syn: {bomb shelter}, {air-raid shelter}]