Surest

adjective

1. Physically secure and certain, non-failing, reliable.

   "This investment is a sure thing.   The bailiff had a sure grip on the prisoner's arm."

2. Certain in one's knowledge or belief.

   "He was sure she was lying.   I am sure of my eventual death.   John was acting sure of himself but in truth had doubts."

3. Certain to act or be a specified way.

   "Be sure to lock the door when you leave."

4. Free from danger; safe; secure.
5. Betrothed; engaged to marry.