In DanMachi's universe, Argonaut is the name of the story of a boy who wants to be a hero, according to the conversation between the Loki familiar members from 17:16 in episode 8:
Bell: This is my adventure. I want to be... I want to be a hero!
Tiona (Tione?): Like the Argonaut...
Finn: It's the story of a boy who wants to be a hero, yeah.
Tiona/Tione: I liked that story, too.
However, that is not its first mention in the story. Argonaut appeared as early as episode 5 as part of the visual, when Bell was answering the question posed by the grimoire, as to what he sought from magic (the image corresponds to the bold line):
Bell: What do I seek from magic?
Bell: The woman who's faster than anyone, like a lightning bolt... I want to catch up to her.
Bell: Is that all?
Bell: If it were possible... I'd like to be a hero. Like in the fairy tales I read when I was a kid. A hero they'd be satisfied with.
Bell: You're such a child.
Bell: I'm sorry. But that's who I am!
Bell: That's who I am!
While the alphabets are distorted, it still bears a lot of similarities to English alphabet, especially A, R, U, T.
I don't know whether the light novel reveals the name of the fairy tale in this scene, though.
The fact that Bell attained the skill Argonaut, which is named after the story of a boy who wants to be hero, signifies his strong desire to become a hero, which is the meaning of the kanji in the title of episode 8 [英雄願望]{Argonaut}.
