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.
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.
Don't bite your nails.
- Ne ronge pas tes ongles.