c.1300, "safe, secure," later "mentally certain" (mid-15c.), from Old French sur, seur "safe, secure," from Latin securus "free from care, untroubled, heedless, safe" (see secure (adj.)). Pronunciation development followed that of sugar. 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 a qualifier meaning "assuredly" goes back to early 15c. Sure-footed is from 1630s; sure thing dates from 1836. In 16c.-17c., Suresby was an appellation for a person to be depended upon.
Yes; certainly: Sure, I'll support you (1842+)