sureyoudaoicibaDictYouDict[sure 词源字典]
sure: [14] Sure and secure are doublets – that is to say, they come from the same ultimate source, but have diverged over the centuries. Latin sēcūrus (etymologically ‘without care’) was borrowed directly into English as secure, but in Old French it evolved into sur, from which English gets sure.
=> secure[sure etymology, sure origin, 英语词源]
sure (adj.)youdaoicibaDictYouDict
early 13c., "safe against attack, secure," later "firm, reliable" (c. 1300); "mentally certain, confident" (mid-14c.); "firm, strong, resolute" (c. 1400), from Old French seur, sur "safe, secure; undoubted, dependable, trustworthy" (12c.), from Latin securus "free from care, untroubled, heedless, safe" (see secure (adj.)). Pronunciation development is that of sugar (n.).

As an affirmative meaning "yes, certainly" it dates from 1803, from Middle English meanings "firmly established; having no doubt," and phrases like to be sure (1650s), sure enough (1540s), and for sure (1580s). The use as an adverb meaning "assuredly" goes back to early 14c. Sure-footed is from 1630s, literal and figurative; sure thing dates from 1836. In 16c.-17c., Suresby was an appellation for a person to be depended upon.