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 ]