Cut to the quick
Quick in this sense means live tissue or flesh as in the biblical context of the quick and the dead, which means the living and the dead, and comes from Acts 10: 42, “to be the judge of quick and dead”. Cut to the quick means to hurt someone deeply, either physically or emotionally and the expression has been around since the 1500s. Shakespeare writes of people being ‘touched’ to the quick in several of his plays, which means the same thing. Biting or cutting one’s fingernails to the quick i.e. the live tissue, comes later during the 19th century.