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.
At some UK schools in the past where the normal uniform for boys included long trousers, one form of punishment was to make an offending student wear short trousers for a period as though they were much younger boys. This punishment would not work in Britain nowadays, where shorts no longer have that connotation and indeed where if anything the demand by pupils is to be able to wear (casual) shorts (in summer) when they are not permitted. But it is sometimes used in the present day in Singapore, where at most secondary schools boys go from shorts uniform to longs at the start of Secondary 3 (normally the year in which they turn 15). Schools recently using this form of discipline include Pasir Ris Secondary School and occasionally Presbyterian High School. These are both co-educational schools and it works as a deterrent because boys of 16 or 17 find it very embarrassing to have to be seen, especially by girls in the same school whom they may be trying to impress, in lower-secondary uniform after being in "adult" longs for perhaps two years. At Pasir Ris the students undergoing this regime are known as "the short pants people".
At Seng Kang Secondary School in Singapore, offending girls must wear a "red vest" for a given number of days in circumstances where boys would receive a caning, since girls may not receive corporal punishment in Singapore schools.