Wikipedia
Rhaphanidosis is the act of inserting the root of a plant of the raphanus genus (commonly known as a radish) into the anus. It is mentioned by Aristophanes as a punishment for adultery in Classical Athens in the fifth and fourth century BC. It is also allegedly a punishment for other sex-related crimes, such as promiscuity and homosexuality. Later classical references to the punishment include Catullus 15 where percurrent raphanique mugilesque (both radishes and mullets will run you through) is threatened against those who cast lascivious eyes on the poet's boyfriend.
There is some doubt as to whether the punishment was ever enforced or whether the reference to it in the debate between Right and Wrong in The Clouds of Aristophanes should be understood as signifying public humiliation in general. However, there are claims that as capital punishment, the items shoved up the anus are as rough as possible, and as many as possible are used until the person dies of internal hemorrhaging.