guarantee (n.) Look up guarantee at Dictionary.com
1670s, alterted (perhaps via Spanish garante), from earlier garrant "warrant that the title to a property is true," early 15c., from Old French garant "defender, protector," from Germanic (see warrant (n.)). For form evolution, see gu-. Originally "person giving something as security;" sense of the "pledge" itself (which is properly a guaranty) developed 17c.
guarantee (v.) Look up guarantee at Dictionary.com
1791, from guarantee (n.). Garanten in this sense is from early 15c. Related: Guaranteed; guaranteeing.