Guarantee
late 17th century (in the sense ‘guarantor’): perhaps from Spanish garante, corresponding to French garant (see warrant), later influenced by French garantie ‘guaranty’.
wiktionary
From Old French guarantie (perhaps via a later Spanish garante), from the verb guarantir(“to protect, assure, vouch for”), ultimately from Old Frankish *warjand, *warand(“a warrant”), or from guaranty. Doublet of guaranty and warranty.
etymonline
guarantee (n.)
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 a Germanic source, from Proto-Germanic *war- "to warn, guard, protect," from PIE root *wer- (4) "to cover." For form evolution, see gu-. Sense of the "pledge" itself (which is properly a guaranty) developed 18c.
guarantee (v.)
1791, "to be surety for," from guarantee (n.). Garanten in this sense is from early 15c. Related: Guaranteed; guaranteeing.
