Chassé-croisé literally describes a 'criss-cross' step in dancing where partners switch places. It's a fantastic and unique French term that you won't hear every day.
In the song, it's used metaphorically in the line "Amour chassé-croisé" (criss-cross love) to perfectly describe the show's tangled romance: Marinette loves Adrien, but as Chat Noir, he loves her superhero alter-ego, Ladybug. This one phrase captures the entire complicated, heart-wrenching dynamic.
Step into the streets of Paris with French singer Lou and guest Lenni-Kim for the ultimate superhero romantic mess. Marinette introduces herself as regular person, "Une fille comme les autres" [A girl like any other], who transforms into the hero Ladybug whenever evil forces take over the city. She teams up with Chat Noir (Black Cat), but their partnership is full of teenage drama. The track literally calls their romantic situation an "Amour chassé-croisé" [Crossed love], an idiom for a classic romantic mix-up where person A likes person B, but person B likes person C. Marinette says plainly, "Mon cœur aime Adrien / Pas ce héros masqué qui m'aime" [My heart loves Adrien / Not this masked hero who loves me]. She is head over heels for a boy named Adrien in everyday life and constantly brushes off her superhero partner, having no clue that Chat Noir and Adrien share the exact same face behind the mask.
Meanwhile, Lenni-Kim jumps in as the voice of Chat Noir, fighting villains while nursing a bruised ego. He admits, "J'ai peur d'aimer pour rien" [I'm afraid of loving in vain], desperate to win Ladybug's heart while she keeps saving the day as a "porte-bonheur" [good luck charm]. Marinette readily admits that out of costume she is clumsy and insecure—"Dans la vraie vie je suis fragile / J'apprends souvent à mes dépens" [In real life I am fragile / I often learn the hard way]—yet she still laughs off her troubles. It is energetic, dramatic, and funny because their entire dilemma would disappear if either hero would take off their mask for five seconds.