Impermeable to air or other gases.
Highly reserved in some matter, particularly tight-lipped or tight-fisted.
Having no weak points or flaws.
Incontrovertible; demonstrably true or certain.
Absolute and without explanation, as in a command from God like "Thou shalt not kill!"
Being a style of argument in which a person presents their reasoning as categorically true, even if it is not necessarily so.