Moth Gonna Fly is a song Trent and Jesse start making up lyrics for during a conversation with Jane in It Happened One Nut. It is unknown if the song was ever finished, but the two seem very enthusiastic about it.
Note: What we hear on the show is the first attempt at coming up with lyrics and a basic vocal melody. This is the transcript of the relevant parts of the dialogue, including rejected lines and discussion about the song.
Trent - "You've hijacked my brain." Jesse - "Moth to a flame." Trent - "If you don't release me..." Jesse - "It'll really be lame." Trent - No. Jesse - "I'll forfeit the game." Trent - Nah. Jesse - "My soul's waves of grain." Trent - I've heard that somewhere before. Jane - You're driving me insane! Trent - Too many syllables.
Jane - Hey! I just remembered! Big string give-away at the guitar store, this way. Trent - "You've hijacked my brain..." Jesse - "Moth to a flame." Trent - "If you don't release me..." Jesse - "I'll go just the same." Both - (rhythmically) "Moth gonna fly, moth gonna fly, moth gonna fly from your love." Jane - Hey, look! Monster trucks and naked models! Naked, naked, naked! Trent - All right, second verse. "It's my personal hell, where I roast in my shell..." Jesse - "Roast in my shell?"