Un beau brin de fille
“Un beau brin de fille” means a pretty, young girl, or a beautiful, seductive woman, or a good looking girl. “Un brin” can mean a blade (of grass), a piece, a sprig (of plant), a little, a tad, a touch of or a bit of. So the idiom literally means something like “a beautiful bit of girl” or more simply, “a beauty.”