FI'BRIN, noun [See Fiber.] A peculiar organic compound substance found in animals and vegetables. It is a soft solid, of a greasy appearance, which softens in air, becoming viscid, brown and semitransparent, but is insoluble in water. It is the chief constituent of muscular flesh.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI