guarantee (n.) Look up guarantee at
1670s, "person that gives security," altered (perhaps via Spanish garante or confusion with legalese ending -ee), from earlier garrant "warrant that the title to a property is true" (early 15c.), from Old French garant "defender, protector; warranty; pledge; justifying evidence," from Germanic (see warrant (n.)). For form evolution, see gu-. Sense of the "pledge" itself (which is properly a guaranty) developed 18c.
guarantee (v.) Look up guarantee at
1791, "to be surety for," from guarantee (n.). Garanten in this sense is from early 15c. Related: Guaranteed; guaranteeing.