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]

WordNet (r) 3.0 (2006):

bulletproof
    adj 1: without flaws or loopholes; "an ironclad contract"; "a
           watertight alibi"; "a bulletproof argument" [syn:
           unassailable, unshakable, watertight,
           bulletproof]
    2: not penetrable by bullets; "bulletproof glass"; "bulletproof
       vest"
    v 1: make bulletproof; "bulletproof the car"

The Jargon File (version 4.4.7, 29 Dec 2003):

bulletproof
 adj.

    Used of an algorithm or implementation considered extremely robust;
    lossage-resistant; capable of correctly recovering from any imaginable
    exception condition ? a rare and valued quality. Implies that the
    programmer has thought of all possible errors, and added code to protect
    against each one. Thus, in some cases, this can imply code that is too
    heavyweight, due to excessive paranoia on the part of the programmer. Syn.
    armor-plated.


The Free On-line Dictionary of Computing (30 December 2018):

bulletproof
armour-plated

   Used of an algorithm or implementation considered extremely
   robust; lossage-resistant; capable of correctly recovering
   from any imaginable exception condition - a rare and valued
   quality.  Synonym armor-plated.

   [Jargon File]