Papillonne comes from the verb papillonner, which literally means "to butterfly" or "to flit around like a butterfly". It's a charming and evocative word that paints a vivid picture.
In the context of this song, it's used metaphorically to describe someone who is not committed and goes from one person to another, much like a butterfly flitting from flower to flower. The line "On sait tous que ton mec papillonne" means "We all know your guy flits around (with other girls)", highlighting his unfaithful nature in a very poetic way.
Have you ever tried to warn a friend about their partner? In "Fake Love," Monsieur Nov sings from the perspective of a friend watching someone they care about get hurt in a bad relationship. The narrator sees all the red flags: her boyfriend doesn't answer her calls, everyone knows he flirts with other people ("papillonne"), and he doesn't give back any of the love she gives. The narrator warns that she is "playing with fire," but the friend won't listen, as if she has a "veil before her eyes."
The song highlights the painful difference between what she wants and what he wants. While she was looking for a real commitment ("te poser"), he was only interested in her for "one night." The narrator predicts the inevitable heartbreak, telling her that her boyfriend will leave her and she will be left devastated ("anéantie"). The powerful chorus repeats the core message: all this suffering is for a love that isn't real, it's just "fake, fake love."