Definition from Wiktionary, the free dictionary
Alternative forms 
- (of a material) Capable of withstanding a direct shot by a bullet fired from a gun.
- A bulletproof window.
- A bulletproof vest.
- (idiomatic) reliable, infallible, sturdy or error-tolerant.
- (usually of an idea or concept) Unbreakable, very tough.
- (infallible): foolproof
capable of withstanding a bullet
- To make proof against bullets.
- (slang) to make resistant to failure.
- We have to bulletproof this program before we let the users at it; check every input, catch every possible flaw...it must not fail in use.