'unbutton' originates from the prefix 'un-' (Old English) combined with the noun 'button', ultimately from Old French 'bouton' (from Late Latin 'bottonem'), where 'un-' meant 'reverse' or 'remove' and 'button' referred to a fastener or knob.

'button' changed from Old French 'bouton' to Middle English 'botoun' and eventually became modern English 'button'; adding the prefix 'un-' created 'unbutton' to express undoing a button.

Initially it meant 'to remove or undo a button'; this basic sense has remained stable into modern usage.