Meanings
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1
verb
to undo the buttons of a garment or other object so that it opens
He began to unbutton his coat as soon as he entered the warm room.
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2
verb
to come open because the buttons have been undone or have slipped free
The stiff collar would not unbutton easily.
Thesaurus
Etymology
From Middle English unbotonen, formed from the reversing prefix un- + button. It literally means to reverse the fastening done by buttons.