Английский Этимологический словарь - sure

- c.1250, "safe, secure," later "mentally certain" (c.1330), from O.Fr. sur, seur "safe, secure," from L. securus "free from care, untroubled, heedless, safe" (see secure). Pronunciation development followed that of sugar. As an affirmative meaning "yes, certainly" it dates from 1842. Sure-footed is from 1633; sure-fire first attested 1901; sure thing dates from 1836.