unbutton

verb2

1. •
   To open (something) by undoing its buttons.

   • Being now too high wound up to bear a delay, he unbutton'd, and drawing out the engine of love-assaults, drove it currently, as at a ready-made breach — 1749
2. •
   To come open by having its buttons unfastened.

   • He was so muscled and firm, and the canvas shirt under his jacket unbuttoned easily. — 2010