ラビリンス means “labyrinth” or “maze,” a dramatic loanword that evokes mystery, confusion, and being unable to find your way out.
In the song, the narrator says he has been drawn into a ラビリンス by the woman’s playful gestures, using the maze as a metaphor for their complicated and irresistible relationship.
Janne Da Arc's "Lunatic Gate" is a song about a man who knows exactly what this relationship is and still can't walk away. He opens on her eyes, "冷めた君の瞳" (your cold eyes), which he says are "相変わらず無機質な宝石みたい" (as always, like an inorganic jewel). She looks down on men, yet she's very good at 上目使い (glancing up through her lashes, the cute upward look). When he starts talking about "僕の彼女~" (my girlfriend...), she pretends not to hear and "僕の口唇にかみついて来る" (bites at my lips). So he's not exactly faithful either. His big promise is to take her to the "Lunatic Gate" and "何か隠してるその扉 開いてみせて" (open that door you're hiding something behind). The gift scene is blunt and a bit much: he buys her a low-cut dress not "着せたいから" (because I want you to wear it) but because he wants to take it off her.
Then it turns. Seeing her outside in daylight throws him: "暗闇より 太陽の下の方が 似合いすぎてる君を 見つめられないのはなぜ?" (why can't I look at you, when you suit the sunlight far more than the dark?). He feels led into a ラビリンス (labyrinth) and steered by her "いたずらな" (mischievous) little gestures. She wakes up in his arms knowing it's a "つかみ所のない関係" (a relationship with nothing to hold on to), and he spots "乾いた涙の跡" (traces of dried tears) under her forced smile. His line "うまく嘘をついて" (lie better) is cold, and it's the best moment in the song, because he admits right after that he used to prefer nights alone and now finds himself "君の感触をさがしてた" (searching for the feel of you).