Bulletproof

adj.resilient, capableprofessional

Meaning 1

Designed to resist penetration by bullets.

Reach for it when describing certified protective material or equipment

The vehicle uses bulletproof panels around the passenger compartment.

everyday
armored
protective hard covering is visible
not when resistance to bullets specifically has been verified
professional
ballistic-resistant
tested protective performance needs accurate terminology
not when general readers need a familiar description
formal
protective
reducing harm is the broad function
not when a defined threat level must be conveyed

So thoroughly prepared or robust that failure appears very unlikely.

Reach for it when offering strong informal praise to a plan, case, or system

The rehearsal exposed two gaps before anyone called the launch plan bulletproof.

playful
airtight
an argument seems to leave no opening
not when a plan or physical system rather than reasoning is praised
playful
rock-solid
informal confidence in reliability is welcome
not when evidence does not justify such emphatic certainty
professional
robust
a system handles stress and variation well
not when near-total confidence is intentionally claimed
formal
failsafe
failure triggers a safe condition
not when general strength rather than a safety mechanism is meant

bulletproof vest
A bulletproof vest must match the threat level and fit the wearer correctly.
bulletproof glass
The counter is separated by certified bulletproof glass.
bulletproof plan
No plan is truly bulletproof, but this one has tested fallbacks.

Figurative bulletproof is deliberate overstatement. Use robust, well-tested, or resilient when uncertainty should remain visible.

The definition and the example on this page are written here. The dictionaries above ground which meaning is being described, not the wording.