unbutton -- (undo the buttons of; "unbutton the shirt")
Overview of adj unbuttoned
The adj unbuttoned has 2 senses
unbuttoned, unfastened -- (not buttoned; "the wind picked up the hem of her unbuttoned coat")
unbuttoned, unlaced -- (not under constraint in action or expression; "this unbuttoned and disrespectful age"- Curtis Bok; "unlaced behavior in the neighborhood pub")