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From The Collaborative International Dictionary of English v.0.48:
Guarantee \Guar"an*tee`\, v. t. [imp. & p. p. guaranteed; p, pr. & vb. n. Guaranteeing.] [From Guarantee, n.] In law and common usage: to undertake or engage for the payment of (a debt) or the performance of (a duty) by another person; to undertake to secure (a possession, right, claim, etc.) to another against a specified contingency, or in all events; to give a guarantee concerning; to engage, assure, or secure as a thing that may be depended on; to warrant; as, to guarantee the execution of a treaty. [1913 Webster] The United States shall guarantee to every State in this Union a republican form of government. --Constitution of the U. S. [1913 Webster]