Keratin is the principal matter of fingernails.
a tough, fibrous, insoluble protein forming the principal matter of hair, nails, horn, etc.
Origin of keratinkerat(o)- + -in
A tough, insoluble protein substance that is the chief structural constituent of hair, nails, horns, and hooves.
Origin of keratinGreek keras, kerāt-, horn; see ker-1 in Indo-European roots + –in.
(countable and uncountable, plural keratins)