a promise or assurance, especially one in writing, that something is of specified quality, content, benefit, etc., or that it will perform satisfactorily for a given length of time: a money-back guarantee.
1679, perhaps via Sp. garante, from O.Fr. guarantie, pp. of fem. guarantir "to protect," from guarant "warrant," from Frank. *warjand-s, from P.Gmc. *war-, from PIE base *wer- "to cover" (see warrant). For form evolution, see gu-. Originally "person