Chrysalide literally translates to "chrysalis," the pupa of a butterfly. It's an unexpected and poetic word to find in a song, especially one with such raw and explicit themes.
In the context of the song, Gainsbourg uses it metaphorically, likely referring to the female body or a part of it, suggesting something delicate, perhaps in a state of transformation or enclosure. The juxtaposition of this beautiful, natural image with the song's otherwise crude language creates a striking and memorable contrast, making it a truly unique and enticing word.
Gainsbourg builds the whole song around one dirty pun on a brand name: "Un glass securit" ["A security glass"] — Sécurit was the trademark on the tempered glass fitted in French windows and car doors — repeated as cover for what he's actually asking, "Pour prendre ton clit" ["To take your clit"]. The verses keep dressing up the same request in borrowed vocabulary, half-classical and half-slang: "Je recherche Aphrodite" ["I'm looking for Aphrodite"], "Que de langues sodomites" ["So many sodomite languages"].
By the last verse he drops the wordplay for real literature, announcing "Mallarmé dixit, je cite" ["Mallarmé dixit, I quote"] and lifting the poet's own erotic image almost whole: "S'avance le palais de cette étrange bouche / Pâle et rose comme un coquillage marin" ["The palace of this strange mouth advances / Pale and pink like a sea shell"]. Trademark or dead poet, it's the same joke — Gainsbourg reaching for any language that lets him say what he wants without quite saying it.