Possible Clues:

• Not allowing air or gas to pass in or out
• Like good alibis
• Jars may be drunk after broadcast
• (Of a defence) having no weaknesses
• Rock-solid, as a case
• Leak-proof boat's ultimate in first class privilege
• Impenetrable
• Following Southern Hungary's defection turbulent high strait is impenetrable
• Without weak points
• Not allowing gas to enter or escape