The Collaborative International Dictionary of English v.0.48:

bullet-proof \bul"let-proof`\, bulletproof \bul"let*proof`\, a.
   1. Capable of resisting the force of a bullet; resistant to
      penetration by a bullet; armored; as, a bulletproof vest;
      a bulletproof window.
      [1913 Webster +PJC]

   2. designed so as to be resistant to abuse or misuse and
      incapable of malfunction under normal use; as, a
      bulletproof computer program.
      [PJC]

   3. so well thought out as to be resistant to criticism or
      certain to succeed; as, a bulletproof plan.
      [PJC]