sure
adj.adj. feeling certain that something is true or will happen. You use this when you have no doubt about a fact or a plan.
adj. confident in the truth or accuracy of a statement; certain of a future outcome. Often used predicatively after a linking verb.
I am sure that I locked the front door.
The manager is sure that the new product will be a success based on early sales figures.
While the evidence was largely circumstantial, the lead investigator remained sure of the suspect's involvement, citing a series of unexplained inconsistencies in the original alibi.
From Middle English sure, seur, sur, from Middle French sur or Old French seür, from Latin sēcūrus (“secure”, literally “carefree”), from sē- (“apart”) + cūra (“care”) (compare Old English orsorg (“carefree”), from or- (“without”) + sorg (“care”)). See cure. Doublet of secure and the now obsolete or dialectal sicker (“certain, safe”). Displaced native Middle English wis, iwis (“certain, sure”) (from Old English ġewis, ġewiss (“certain, sure”)), as well as Middle English siker (“sure, secure”) (from Old English sicor (“secure, sure”)) with which it was cognate.
Commonly followed by a 'that' clause or the preposition 'of'. When used as an exclamation, it functions as an informal affirmative.