A punishment outfit is a way of dressing that is reserved for punishment.
Some punishment outfits might be a specific item that would not otherwise be unusual save for it has been set aside for this purpose—like a pair of red shorts—or it could be clothes they might not otherwise voluntarily wear. Punishment outfits work via shame and humiliation and are thus a form of psychological punishment.
This first effect is the wearer knows that they are wearing it as a punishment so it becomes a symbol of their shame. If the meaning of the outfit is not obvious then there is worry others might ask why they are wearing the item, this becomes especially in the case of minors who are still growing, as the item might be kept past the normal replacement stage, making the question more likely.
The punishment outfit can also have special meanings, such as red shorts or skirt to symbolize an impending spanking, or clothes that are more age appropriate for a younger age group such a schoolchild styles or even diapers become comments on lack of maturity. Items like a donkey or dunce cap symbolize the lack of desired traits like intelligence.