Other forms: unbuttoned; unbuttoning; unbuttons
When you unbutton something, you unfasten or open it, the way you unbutton your coat before you take it off.
To undo the buttons on a piece of clothing is to unbutton it. If your formal clothes feel uncomfortable, you might take off your tie and unbutton your top shirt button. You can also figuratively unbutton, by becoming more relaxed and uninhibited: "Kick off your shoes and unbutton a little — we're at the beach!" Unbutton comes from adding un-, "opposite of," to the verb button, "fasten with buttons."