Crossword Dictionary
sure
certain, sure - a
having or feeling no doubt or uncertainty; confident and assured; "felt certain of success"; "was sure (or certain) she had seen it"; "was very sure in his beliefs"; "sure of her friends"
exercising or taking care great enough to bring assurance; "be certain to disconnect the iron when you are through"; "be sure to lock the doors"
Being sure has to do with being certain, convinced, trustworthy, or dependable. You can trust a sure thing.
Besides being a synonym for yes, sure has to do with lots of kinds of dependability. If you have a sure eye, then you have good aim, maybe with shooting a basketball or throwing darts. If you're sure of yourself, you're confident. A sure thing or sure bet is something almost certain to happen. If you're 100% convinced of something, you're sure of it.