From Longman Dictionary of Contemporary Englishunbuttonun‧but‧ton /ʌnˈbʌtn/ verb [transitive] to undo the buttons on a piece of clothing OPP button up He took off his sweater and unbuttoned his shirt.→ See Verb table
Examples from the Corpus
unbutton• I adjusted my tie and buttoned up the coat which, a little while later, I would be unbuttoning.• She slowly unbuttoned her blouse.• Father Poole began to unbutton his overcoat.• He pulled down his necktie and unbuttoned his shirt.• Then I felt his hand unbuttoning my blouse.• She started to unbutton my camisole.• The two other birds had jumped on me and started unbuttoning my flies.• It takes Redford and Pfeiffer close to an hour to begin unbuttoning one another.• Smiling at the idea, Katherine turned away from the window, unbuttoning the pyjamas, heading towards her bathroom. 