unfastened
On Danbooru this will typically refer to a garment such as a bra which has become unfastened and hangs somewhat loosely while the wearer is undressing. One assumes that the attachment is made by unseen hook.
For clothes that specifically use buttons, zippers or knots, see unbuttoned, unzipped and untied, respectively.
The following tags are aliased to this tag: unhooked (learn more).