Etymologicum Magnum

“Radamanthus: [the origin of this name] is either that he went crazy [emanê] over roses [roda] trampled by a bull or that he was educated [epaideuthê*] among the Trojans in Rhodes.”

 ῾Ραδάμανθυς: ῍Η ὅτι περὶ τὰ ῥόδα ἐμάνη τὰ ὑπὸ τοῦ ταύρου προϊέμενα, ἢ ὅτι ἐν ῾Ρόδῳ παρὰ ταῖς ᾿Ιλιάδεσιν ἐπαιδεύθη.

*The author is thinking of some form of manthanô here to get –manthus.

I have yet to find explanations for either of these accounts. So, to make up for it, here are some other stories about Rhadamanthus.

Dio. Sic. 5.78-9

“They claim that Radamanthus provided the most just judgments of all and also imposed the most inflexible punishment for raidings, and sacrilege, and other wicked deeds. He also is said to have established [these laws] in not a few islands and much of the land of Asia near the sea, since they were willing to put themselves in his hands thanks to his sense of justice. They also claim that Rhadamanthus handed the kingship over to one of his own children, Eruthros, from whom the Eruthrians were named.

People also say that Khios entrusted itself to Oinopiôn, the son of Ariadne, Minos’ daughter whom some claim was Dionysus’ son and learned from his father everything about wine-making. They also claim that Rhadamanthus granted a city or island to each of the leaders around him: he gave Lemnos to Thoas, Kurnos to Enuos, Peparêthos to Staphulos, Marôneia to Euanthos, Paros to Alkaios, Dêlos to Aniônos, and Andros, which was named for him, to Andros. Thanks to the exaggeration of his sense of justice, people have told the myth that he was made a judge in Hades and he distinguished the righteous men from the wicked. This same honor has been given to Minos, since he ruled most lawfully and was especially solicitous of justice. The third brother was Sarpedon.”

          ῾Ραδάμανθυν δὲ λέγουσι τάς τε κρίσεις πάντων δικαιοτάτας πεποιῆσθαι καὶ τοῖς λῃσταῖς καὶ ἀσεβέσι καὶ τοῖς ἄλλοις κακούργοις ἀπαραίτητον ἐπενηνοχέναι τιμωρίαν. κατακτήσασθαι δὲ καὶ νήσους οὐκ ὀλίγας καὶ τῆς ᾿Ασίας πολλὴν τῆς παραθαλαττίου χώρας, ἁπάντων ἑκουσίως παραδιδόντων ἑαυτοὺς διὰ τὴν δικαιοσύνην. τὸν δὲ ῾Ραδάμανθυν ᾿Ερύθρῳ μὲν ἑνὶ τῶν αὑτοῦ παίδων παραδοῦναι τὴν βασιλείαν τῶν δι’ ἐκεῖνον ᾿Ερυθρῶν ὀνομασθεισῶν, Οἰνοπίωνι δὲ τῷ ᾿Αριάδνης τῆς Μίνω Χίον ἐγχειρίσαι φασίν, ὃν ἔνιοι μυθολογοῦσι Διονύσου γενόμενον μαθεῖν παρὰ τοῦ πατρὸς τὰ περὶ τὴν οἰνοποιίαν. τῶν δ’ ἄλλων τῶν περὶ αὐτὸν ἡγεμόνων ἑκάστῳ νῆσον ἢ πόλιν δωρήσασθαι λέγουσι τὸν ῾Ραδάμανθυν, Θόαντι μὲν Λῆμνον, ᾿Ενυεῖ δὲ Κύρνον, Σταφύλῳ δὲ Πεπάρηθον, Εὐάνθει δὲ Μαρώνειαν, ᾿Αλκαίῳ δὲ Πάρον, ᾿Ανίωνι δὲ Δῆλον, ᾿Ανδρεῖ δὲ τὴν ἀπ’ ἐκείνου κληθεῖσαν ῎Ανδρον. διὰ δὲ τὴν ὑπερβολὴν τῆς περὶ αὐτὸν δικαιοσύνης μεμυθολογῆσθαι δικαστὴν αὐτὸν ἀποδεδεῖχθαι καθ’ ᾅδου καὶ διακρίνειν τοὺς εὐσεβεῖς καὶ τοὺς πονηρούς. τετευχέναι δὲ τῆς αὐτῆς τιμῆς καὶ τὸν Μίνω, βεβασιλευκότα νομιμώτατα καὶ μάλιστα δικαιοσύνης πεφροντικότα. τὸν δὲ τρίτον ἀδελφὸν Σαρπηδόνα

Apollodorus 2.64

“[Linos] was the brother of Orpheus. After he arrived at Thebes and became Theban, he was struck with his cithara by Herakles and died. Herakles killed him because he was angry at him for striking him. When some were demanding that he pay the penalty for murder, Herakles invoked the law of Radamanthus which said that whoever defended himself against a man who began the injustice was immune to punishment. Thus he was acquitted.”

οὗτος δὲ ἦν ἀδελφὸς ᾿Ορφέως· ἀφικόμενος δὲ εἰς Θήβας καὶ Θηβαῖος γενόμενος ὑπὸ ῾Ηρακλέους τῇ κιθάρᾳ πληγεὶς ἀπέθανεν· ἐπιπλήξαντα γὰρ αὐτὸν ὀργισθεὶς ἀπέκτεινε. δίκην δὲ ἐπαγόντων τινῶν αὐτῷ φόνου, παρανέγνω νόμον ῾Ραδαμάνθυος λέγοντος, ὃς ἂν ἀμύνηται τὸν χειρῶν ἀδίκων κατάρξαντα, ἀθῷον εἶναι, καὶ οὕτως ἀπελύθη.