็ดไธ้ literally means "one sheet of paper," expressing an extremely narrow difference or boundary between two things. In the song, it describes pleasure and danger moving across a razor-thin line, making this poetic idiom especially memorable and emotionally charged.
"Vanity" by Janne Da Arc is a song about loving someone who belongs to somebody else, sung by the person stuck in the middle. The narrator knows this woman through nights together, and he admits that in bed he gets "ๅฐใใ ใๆๅฐๆชใซใชใ" [a little bit mean]. He also tells us straight out why he never asks about the rest of her life: he is "็พๅฎ ใใ้ใใฆใใใ ใ" [just running away from reality], because he can already see how it ends. His wish is simple and a bit desperate: "ๆใๆฅใชใใใฐใใใฎใซ" [I wish the morning would never come].
The detail that sticks is the ring. "ๅทฆใใใๆใซ" [on her left ring finger] sits "ๅ ใ็ฎใใใใชๆ่ผช" [a shining, eyesore of a ring], and because of it "Kissใ่ฆใ" [the kiss tastes bitter]. ใใใๆ literally means "medicine finger", the everyday Japanese word for the ring finger, so the line is clearly pointing at a wedding or engagement ring. Then he asks the question he hates asking: "ไปๅคใฏๅฝผใฎ่ ใฎไธญใงๅคขใ่ฆใใฎ?" [Will you dream in his arms tonight?]. He even wonders if she is just tired of "็ซ้ใณ" [playing with fire], the Japanese idiom for a risky affair. My favourite moment is the last line, which is quietly brutal: "่งฆใใใๅบฆใซ ๅใ้ ใใธ่ฆใใ" [every time we touch, you look further away].