surest
adjective
Definitions
Physically secure and certain, non-failing, reliable.
Example: This investment is a sure thing. The bailiff had a sure grip on the prisoner's arm.
Certain in one's knowledge or belief.
Example: He was sure she was lying. I am sure of my eventual death. John was acting sure of himself but in truth had doubts.
Certain to act or be a specified way.
Example: Be sure to lock the door when you leave.
Free from danger; safe; secure.