Chevilles Molly refers to "Molly anchors," which are a specific type of wall plug used for plasterboard. This term is quite specific to construction.
In the song, Vald uses this literal meaning to set up a double entendre. While he talks about working with his father on a construction site using Molly anchors, he then transitions to a more metaphorical and explicit use of "Molly" in relation to a woman, playing on the sound and implying a destructive impact, as her "ke-shné" (a slang term for vagina) was "démolie" (demolished). This clever wordplay makes it memorable and central to the song's theme.
This is one of the shortest tracks on Vald's album NQNT33, and it barely tells a story so much as loop one word until it means two things at once. It opens on a repeated "Oui, oui, oui" ["Yes, yes, yes"], then settles into a single memory: working construction with his father, "Avec mon père, sur le chantier" ["With my father, on the site"], driving in "On prenait des chevilles Molly" ["We were using Molly anchors"] for the plasterboard, "Molly pour le placo" ["Molly for the drywall"]. A Molly is a real French building-supply term for a hollow-wall anchor bolt, and Vald spends the rest of the track addressing "Molly" like a person instead of a piece of hardware: "Molly, quand j'l'ai laissée" ["Molly, when I left her"], "Maudit soit l'jour sans Molly" ["Cursed be the day without Molly"].
He never picks a side of the pun. The hook keeps folding "Molly" back into itself, first the tool his father handed him, then someone he's mourning, until the two blur completely. By the closing "Molly, Molly" the word has stopped meaning either thing specifically and just sits there, repeated, like a name said one too many times.