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.
The United States shall guarantee to every State in
this Union a republican form of government.
of the U. S.