ensure (v.)

mid-15c., insuren, spelling variant of ensuren "to assure, give formal assurance" (late 14c.), also "make secure, make safe" (c. 1400), from Anglo-French enseurer, Old French ensurer, from en- "make" (see en- (1)) + seur, sur "safe, secure, undoubted" (see sure (adj.)).

The particular commercial senses of "make safe against loss by payment of premiums; undertake to ensure against loss, etc." are from mid-17c. (replacing assure in that meaning). Related: Insured; insuring.

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.

word-forming element meaning "in; into," from French and Old French en-, from Latin in- "in, into" (from PIE root *en "in"). Typically assimilated before -p-, -b-, -m-, -l-, and -r-. Latin in- became en- in French, Spanish, Portuguese, but remained in- in Italian.

Also used with native and imported elements to form verbs from nouns and adjectives, with a sense "put in or on" (encircle), also "cause to be, make into" (endear), and used as an intensive (enclose). Spelling variants in French that were brought over into Middle English account for parallels such as ensure/insure, and most en- words in English had at one time or another a variant in in-, and vice versa.