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]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "unbutton":
   cast off, detach, disengage, doff, free, liberate, loose, loosen,
   release, remove, take off, unbar, unbind, unbolt, unbuckle,
   unchain, unclasp, undo, unfasten, unfetter, unglue, unlace,
   unlatch, unleash, unlock, unloose, unloosen, unstick, unstrap,
   untie, weigh anchor