Just like a music instrument without frets someone would be call fretless by having no constrait, limitation, or restriction. Saying someone is fretless is a sexy and colourful way of saying he or she seems free in a cool way.
I hear you're throwing a banger tonight, looks like Imma be fretless again!
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi