Scholia to Lykophron’s Alexandra, Introduction
“In addition to these, here are the characteristics of prominent poets, the lyric ones who sing their songs to a lyre and who may have a chorus of fifty men set up in a circle, those who also used to take a bull as a prize. These features are shared with the dithyrambic poets. The dithyrambic poets are in the habit of composing their fine hymns do Dionysus and they used to take tripods [as gifts?]. These poems are called dithyramboi thanks to the “two exit doors” of Dionysus, Semele’s stomach and Zeus’ thigh. “
καὶ ταῦτα μὲν τὰ γνωρίσματα τῶν καλουμένων κατ’ ἐξοχὴν ποιητῶν, λυρικῶν δὲ γνωρίσματα τὸ πρὸς λύραν τὰ τούτων ἄδεσθαι μέλη καὶ χορὸς ἑστὼς κυκλικῶς ἄνδρας ἔχων πεντήκοντα, οἵπερ καὶ δῶρον ταῦρον ἐλάμβανον.
καὶ διθυραμβικοῖς δὲ τοῦτο κοινόν. οἱ διθυραμβικοὶ δὲ τῶν λυρικῶν εἶχόν τι πλέον τὸ πρὸς τὸν Διόνυσον πολυστρόφους πλέκειν τοὺς ὕμνους καὶ τρίποδας ἐλάμβανον διὸ καὶ διθύραμβοι ἀπὸ τοῦ Διονύσου ἐλέγοντο τοῦ διὰ δύο θυρῶν βάντος, τῆς τε γαστρὸς τῆς Σεμέλης καὶ τοῦ μηροῦ τοῦ Διός.
If you didn’t get the joke, it is because di-thura-ba- [here, duo-thuron-bantos; “two-doors-walking”] presents the essential sounds of dithyramb. Byzantine etymological text repeats the origin and explains it a bit, not without adding another on its own.
Etymologicum Magnum, s.v. dithyrambos
“Dithyrambos: Dionysus. It is an epithet of Dionysus because he was raised in a cave with two doors in Nussê. This is also the hymn named for the god and dedicated to him. It comes from “coming through two doors”, the womb of his mother Semele and Zeus’ thigh—since he was born twice: once from his mother, and once from Zeus’ thigh. This is how he exited the ‘door’ twice.”
Διθύραμβος: ῾Ο Διόνυσος. ᾿Επίθετόν ἐστι τοῦ Διονύσου, ὅτι ἐν διθύρῳ ἄντρῳ τῆς Νύσσης ἐτράφη· καὶ ὁμωνύμως τῷ θεῷ ὁ εἰς αὐτὸν ὕμνος. ῍Η ἀπὸ τοῦ δύο θύρας βαίνειν, τήν τε κοιλίαν τῆς μητρὸς Σεμέλης, καὶ τὸν μηρὸν τοῦ Διός· ἀπὸ τοῦ δεύτερον τετέχθαι, ἀπό τε τῆς μητρὸς, καὶ ἀπὸ τοῦ μηροῦ τοῦ Διός· ἵν’ ᾖ ὁ δὶς θύραζε βεβηκώς.