n.1 an oil from the seeds of a plant, Ricinus communis, used as a purgative and lubricant.
2 (in full castor oil plant) this plant.
Phrases and idioms:
castor oil bean (or castor bean) a seed of the castor oil plant.
Etymology: 18th c.: orig. uncert.: perh. so called as having succeeded CASTOR(2) in the medical sense