Assure

google

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late Middle English: from Old French assurer, based on Latin ad- ‘to’ (expressing change) + securus (see secure).

wiktionary

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From Old French asseurer (Modern French assurer), from Latin ad- + securus(“secure”). Cognate with Spanish asegurar. Doublet of assecure.

etymonline

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assure (v.)

late 14c., "reassure, give confidence to; make secure or safe, protect; bind by a pledge, give a promise or pledge (to do something)," from Old French asseurer "to reassure, calm, protect, to render sure" (12c., Modern French assurer), from Vulgar Latin *assecurar, from assimilated form of Latin ad "to" (see ad-) + securus "safe, secure" (see secure (adj.)). Related: Assured; assuring.