The Collaborative International Dictionary of English v.0.48:

Guaranty \Guar"an*ty\, n.; pl. Guaranies. [OF. guarantie,
   garantie, F. garantie, OF. guarantir, garantir, to warrant,
   to guaranty, E. garantir, fr. OF. guarant, garant, a
   warranter, F. garant; of German origin, and from the same
   word as warranty. See Warrant, and cf. Warranty,
   Guarantee.]
   In law and common usage: An undertaking to answer for the
   payment of some debt, or the performance of some contract or
   duty, of another, in case of the failure of such other to pay
   or perform; a guarantee; a warranty; a security.
   [1913 Webster]