The Collaborative International Dictionary of English v.0.48:
Unbelt \Un*belt"\ ([u^]n*b[e^]lt"), v. t. [1st pref. un- + belt.] To remove or loose the belt of; to ungird. [1913 Webster]WordNet (r) 3.0 (2006):
unbelt v 1: undo the belt of; "unbelt your trousers" [ant: belt]