Having no weak points or flaws.
Highly reserved in some matter, particularly tight-lipped or tight-fisted.
Impermeable to air or other gases.
Physically secure and certain, non-failing, reliable.
Certain to act or be a specified way.
Certain in one's knowledge or belief.
Yes, expressing noncommittal agreement or consent.
Yes; of course.
You're welcome; polite response to being thanked.
Without doubt, certainly.