%guaranty%

v. t. [ imp. & p. p. Guarantied p. pr. & vb. n. Guarantying. ] [ 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, v. t. [ 1913 Webster ]

☞ 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. [ 1913 Webster ]