unbutton

verb

Etymology: From Middle English onbottonen, unbotonen; equivalent to un- + button.

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”

   “"Mr Baxter, I think?" said the latter. He had laid aside his dripping umbrella and was unbuttoning overcoat and coat to reach an inner pocket.”

2. To come open by having its buttons unfastened.

   “He was so muscled and firm, and the canvas shirt under his jacket unbuttoned easily.”