insureyoudaoicibaDictYouDict[insure 词源字典]
insure: [15] Insure and ensure [14] are ultimately the same word. And their common ancestor started out, in fact, as a variant of assure [14]. This came via Old French asseurer from Vulgar Latin *assēcūrāre, a compound verb formed from the Latin prefix ad- ‘to’ and the adjective sēcūrus ‘safe’ (source of English secure and sure).

Anglo-Norman had a variant form, enseurer, which produced English ensure. From fairly early on this had been alternatively spelled insure (using the Latinate prefix in-), but it was not until the 17th century that this version became established in the sense ‘provide cover against loss, damage, etc’ (for which previously the more usual term had actually been assure).
=> assure, ensure, secure, sure[insure etymology, insure origin, 英语词源]
insure (v.)youdaoicibaDictYouDict
mid-15c., insuren, spelling variant of ensuren (see ensure). Took on its particular sense of "make safe against loss by payment of premiums" from mid-17c. (replacing assure in that meaning). Related: Insured; insuring.