Garce is a French word that can be translated as "bitch" or "mean girl." It's a strong, somewhat provocative term, making it memorable and less common in everyday song lyrics.
In the context of Serge Gainsbourg's song, he uses it to describe his lover's unpredictable nature: "Ce que tu peux être garce quand tu t'y mets" (How much of a bitch you can be when you put your mind to it). This word choice highlights the intensity and complexity of their relationship, adding a raw, emotional layer to the lyrics.
Serge Gainsbourg's half-spoken delivery turns "Quand tu t'y mets" ("when you get into it") into a wry catalogue of one woman's extremes: beautiful, cruel, and wounding, all inside a handful of repeated lines. Each verse follows the same shape, a single quality he can't look away from, closed off by "Tu peux pas savoir" ("you have no idea"), as if he's still arguing his case to someone who stopped listening long ago.
By the final verse the tense slides into the past, "notre amour" ("our love"), and the refrain itself changes to "Tu dois plus savoir" ("you must not know anymore") — the same teasing list of what she used to do, now spoken as a quiet admission that whatever they had is finished.