insure (v.)

mid-13c., seur, "safe against attack, secure, out of danger," later "reliable, fit or worthy to be depended upon" (c. 1300); "mentally certain, confident of one's positions" (mid-14c.); "firm, strong, resolute" (c. 1400). It is from Old French seur, sur "safe, secure; undoubted, dependable, trustworthy" (12c.), which is from Latin securus "free from care, untroubled, heedless, safe" (see secure (adj.)).

The pronunciation development is that of sugar (n.). The colloquial pronunciation "sho" is attested by 1871 in representations of U.S. Black speech (fo sho); compare mo.

As "certain to be or happen," 1560s. As an affirmative meaning "yes, certainly" it dates from 1803. This traces back to the Middle English meanings "firmly established; having no doubt," and to phrases such as to be sure (1650s), sure enough (1540s), and for sure (1580s).

To make sure "establish beyond doubt" is from 14c.; be sure "be certain, do not fail" (colloquial, be sure to click the links) is by 1590s. Figurative formulas of certainty beginning as sure as are attested from late 14c. (as fire is red); as sure as (one is) born is from 1640s.

The use as an adverb meaning "assuredly, certainly" is from early 14c. Sure thing "something beyond the possibility of failure, a certainty," is by 1836.

late 14c., from Anglo-French enseurer, from en- "make" (see en- (1)) + Old French seur "sure" (see sure); probably influenced by Old French asseurer "assure." Compare insure. Related: Ensured; ensures; ensuring.