For characters deprived of the hat they were supposedly wearing a short while before the image. Said hat has to be visible somewhere to imply this, otherwise, use no_hat instead.
Basically for any situation where a post is tagged hat, yet the character it belongs to is not wearing it. If he or she is holding it, tag holding hat as well.
The following tags are aliased to this tag: hat_off (learn more).
This tag implicates hat and headwear_removed (learn more).