impervious (adj.)

• airtight
• bombproof
• bulletproof
• burglarproof
• callous
• calloused
• case-hardened
• fire-resisting
• fireproof
• flameproof
• flinty
• foolproof
• hard
• hardened
• hardhearted
• hermetic
• impassable
• impenetrable
• impermeable
• inaccessible
• inflexible
• insensitive
• inured
• leakproof
• obstinate
• pachydermatous
• proof
• resistant
• rustproof
• shatterproof
• shellproof
• soundproof
• steely
• stony
• thick-skinned
• tight
• un-come-at-able
• unaccessible
• unapproachable
• unattainable
• unavailable
• undiscoverable
• unimpressionable
• unmovable
• unobtainable
• unpassable
• unpersuadable
• unprocurable
• unreachable
• unreceptive
• unresponsive
• unsusceptible
• unyielding
• watertight
• weatherproof