Refers to the plant Ricinus communis, the source of castor oil. The kanji 蓖麻 is technical and rarely seen; the word is usually written in kana.
Castor oil is extracted from the seeds of the castor oil plant.
castor bean
The reading ひま is of uncertain origin. The kanji 蓖麻 are Chinese-derived characters specifically used for the castor oil plant.