castor (n.) Look up castor at Dictionary.com
late 14c., "beaver," from Old French castor (13c.), from Latin castor "beaver," from Greek Kastor, literally "he who excels," name of one of the divine twins (with Pollux), worshipped by women in ancient Greece as a healer and preserver from disease.

His name was given to secretions of the animal (Latin castoreum), used medicinally in ancient times. (Through this association his name replaced the native Latin word for "beaver," which was fiber.) In English, castor is attested in this sense from c.1600. Modern castor oil is first recorded 1746; it is made from seeds of the plant Ricinus communis but supposedly possesses laxative qualities (and taste) similar to those of beaver juice, and thus so named.