Airtight
Part of speech: adjective
Definition: Being impermeable to air or other gases. Having no weak points or flaws (as in an "airtight" argument).
Invulnerable
Part of speech: adjective
Definition: Incapable of being wounded, or of receiving injury; not vulnerable.Unanswerable; irrefutable; unable to be damaged by an attack or convinced; as, an invulnerable argument.