Lyric discussion by sweetsunray 

Cover art for Je T'Aime... Moi Non Plus lyrics by Serge Gainsbourg

The meaning of "reins"

rein - means kidney but "les reins" is plural form and means loins. They don't use the single world "rein" for "loin" in French. Of course in a total different context "les reins" may mean "kidneys" in plural too, but obvously not in this context.

French is a language where the context determines the specific meaning of a word, whereas in English it's the opposite and one word creates a whole context. It's the reason why French is a rather wordy language... the context must be created before the meaning can come through.

@sweetsunray That's entirely true. Context is everything for a word to really fulfil its meaning. French is notorious for such finnicky use of such things, but it's also why it's even more perfect for that thing in itself