Synonyms for Airtight (same or very similar meaning)
WordNet sense 1 (having no weak points):
air-tightWordNet sense 2 (immune to attack; impregnable):
invulnerableWordNet sense 3 (not allowing air or gas to pass in or out):
gas-tightWordNet sense 4 (of such close construction as to be impermeable):
tightFrom the ODE community, based on WordNetadd/edit