bul·let·proof 
(blĭt-prf′)
(blĭt-prf′)adj.
1. Impenetrable by bullets.
2. Informal Impervious to assault, damage, or failure; guaranteed: a bulletproof method; bulletproof arguments.
tr.v. bul·let·proofed, bul·let·proof·ing, bul·let·proofs
To make impenetrable by bullets.