simple past and past participle of unbutton
I unbuttoned my shirt to take it off.
Having the buttons undone.
Once a white man in an unbuttoned uniform, camping on the path with an armed escort of lank Zanzibaris, very hospitable and festive, not to say drunk.
From un- + buttoned. In some instances possibly from unbutton.
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In disarray.
Informal, casual.