Guaranty
Guaranty , noun
[Old French guarantie, garantie, French garantie, Old French guarantir, garantir, to warrant, to guaranty, English garantir, from Old French guarant, garant, a warranter, French garant; of German origin, and from the same word as warranty. See Warrant, and compare 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.
Guaranty , transitive verb
[From Guaranty, n.]
In law and common usage: To undertake or engage that another person shall perform (what he has stipulated); to undertake to be answerable for (the debt or default of another); to engage to answer for the performance of (some promise or duty by another) in case of a failure by the latter to perform; to undertake to secure (something) to another, as in the case of a contingency. See Guarantee, transitive verb
Guaranty agrees in form with warranty. Both guaranty and guarantee are well authorized by legal writers in the United States. The prevailing spelling, at least for the verb, is guarantee.