What does he do with the Mishna?
I think the Ramban ties together doing it for the sake of the person as in cutting nails and hair with cutting for the sake of what you get, as in wool. But you can argue that biting nails is for neither, which will make it Eino Tzricha Legufa which is a Derabanan according to Rebbi Shimon. Couple that with the Derabanan of not doing it Bikeli. Makom Tzaar…