Aristophanes, Clouds 1083-104
Just Argument: “What if he should have a radish shoved up his ass because he trusted you and then have hot ashes rip off his hair? What argument will he be able to offer to prevent himself from having a gaping-anus?”
Δίκαιος Λόγος: τί δ᾽ ἢν ῥαφανιδωθῇ πιθόμενός σοι τέφρᾳ τε τιλθῇ,
ἕξει τινὰ γνώμην λέγειν τὸ μὴ εὐρύπρωκτος εἶναι;
The scholion usefully explains “[having a radish shoved up the ass] was the way they punished adulterers when they were caught. They would grab them, and shove radishes into their anuses, and then they would spread hot ash over them, ripping out their hair, working it in for sufficient torments.”
τί δ’ ENM ἢν ῥαφανιδωθῇ RENM: τοὺς ἁλόντας μοιχοὺς οὕτω ᾐκίζοντο· ῥαφανῖδας λαμβάνοντες καθίεσαν εἰς τοὺς πρωκτοὺς τούτων, καὶ παρατίλλοντες αὐτοὺς τέφραν θερμὴν ἐπέπασσον βασάνους ἱκανὰς ἐργαζόμενοι. RVENMNp
And there is another explanation:
Tzetzes, Commentary on the Clouds
“Adulterers, if they were rich and caught could resolve the issue with money; but they really got vengeance against poor men: they took them out publicly in the middle of the agora and burned the hair from their balls by working in hot ash from the fire and they shoved the length of radishes deep into their rectums many times.”
τί δ’ ἢν ῥαφανιδωθῇ: μοιχοὶ πρὶν οἱ πλούσιοι ἁλισκόμενοι ἀπελύοντο χρήμασι, τοὺς πένητας δὲ πανδήμως τιθέμενοι μέσον τῆς ἀγορᾶς τὰς τῶν διδύμων τρίχας ἀνέσπων ἐκ πυρὸς θερμὴν στακτὴν ἐπιπάττοντες καὶ ῥαφανίδων οὐραῖα ἐνέβαλλον εἰς τὰς ὀπὰς τῶν πρωκτῶν ἄλλα πολλὰ προτιμωρησάμενοι.
UPDATE: I have been wondering, “why radishes”? This made me remember a ridiculous etymology Palaiophron posted from Athenaeus
RADISHES: These are so called because they “easily appear” (“raidios phainesthai”).
ΡΑΦΑΝΙΔΕΣ. αὗται κέκληνται διὰ τὸ ῥᾳδίως φαίνεσθαι.
Based on the testimony for this particular punishment (appearing almost exclusively to explain Aristophanes), I suspect that it is a bit of a legend. Radishes appear a lot in comedy (Aristophanes and the fragments). There is something intrinsically funny about them….