Adjective

1. Capable of withstanding a direct shot by a bullet fired from a gun.

   A bulletproof window.

   A bulletproof vest.

2. reliable, infallible, sturdy or error-tolerant.
3. Unbreakable, very tough.

Verb

1. To make proof against bullets.
2. 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.