Physically secure and certain, non-failing, reliable.
Certain in one's knowledge or belief.
(followed by a to infinitive) Certain to act or be a specified way.
(obsolete) Free from danger; safe; secure.
(obsolete) Betrothed; engaged to marry.
(modal adverb) Without doubt, certainly.
(archaic) Without fail, surely.
Yes, expressing noncommittal agreement or consent.
Yes; of course.
You're welcome; polite response to being thanked.