You have dirt under your fingernails.
- Tu as de la crasse sous les ongles.
Biting your fingernails is a bad habit.
- Ronger tes ongles est une mauvaise habitude.
Nail polish remover stinks a lot.
- Le dissolvant à ongles pue beaucoup.
He managed to cure himself of his habit of nibbling his nails.
- Il a réussi à se guérir de son habitude de se ronger les ongles.