| A punishment for adulterers in ancient Athens. Actually, a cuckold could do more or less anything they liked if they caught an adulterer, including death, a fine, or doing nasty things to their genitals, but one established practice was to make an example of them by sticking a radish up them.
The interesting thing is that they even had a word for it. Well, Aristophanes did: he had lots of lovely words for all sorts of weird things. 'Radish' was ραφανος rhaphanos, and 'to stick up a radish up the fundament, as a punishment for adultery' was ραφανιδοω rhapanidoô. This occurs in The Clouds (in the passive, ραφανιδωθηι rhaphanidôthêi), where the Just Argument is trying to convince the Unjust Argument that adultery cannot be committed with impunity.
Catullus also refers to the practice, in his poem 15, line 19, warning one Aurelius to keep his hands off a certain little boy.
There is also a word aporaphanidosis, a much-needed noun meaning the practice of sticking a radish where it won't photosynthesize, etc. | 
