The Collaborative International Dictionary of English v.0.48:
Unbutton \Un*but"ton\, v. t. [1st pref. un- + button.] To loose the buttons of; to unfasten. [1913 Webster]WordNet (r) 3.0 (2006):
unbutton v 1: undo the buttons of; "unbutton the shirt" [ant: button]