A shirt or blouse that has been unbuttoned and left hanging open.
Another possibility is that the shirt has been unbuttoned only in one particular part of the shirt/blouse, e.g. to see the breasts of a girl.

This tag can be also used for open robes (yukata and kimono), bodysuits that have been unzipped, and other types of clothing that are opened in a similar way.

(Partially copied from Danbooru)