Other forms: unbuttoned, unbuttoning, unbuttons
To unbutton is to undo the round fasteners of a garment. Unbutton your coat, unbuttoned at the collar, unbutton the shirt.
It is entirely literal in ordinary use. Unbuttoned as an adjective has a second sense meaning relaxed and unguarded — an unbuttoned interview, unbuttoned prose — which is a pleasant metaphor from loosening a collar. Note that buttons themselves were decorative for centuries before anybody thought of the buttonhole, which arrived in Europe in the thirteenth century.
unbutton is an undoing.
Middle English unbotonen is un- plus button, from Old French bouton, a bud, from a verb meaning to push. A button is something that pushes out.
Middle English
onbottonen
English, today
unbutton
From Middle English onbottonen, unbotonen; equivalent to un- + button.
Etymology from Wiktionary, CC BY-SA