Insure
late Middle English (in the sense ‘assure someone of something’): alteration of ensure.
wiktionary
Recorded since about 1440, as a variant of Middle English ensuren (from Anglo-Norman enseurer, itself from en-(“make”) + seür(“sure”), probably influenced by Old French asseürer(“to assure”)); took on its particular sense of "make safe against loss by payment of premiums" in 1635, replacing assure.
etymonline
insure (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.
