Originally: the plant sweet flag, Acorus calamus. In later use: any plant of the genus Acorus (family Acoraceae), the members of which are rushlike flowering plants with inconspicuous flowers, native chiefly to wetlands in Europe and Asia. Also (in form Acorus): the genus itself.
Late Middle English; earliest use found in Guy de Chauliac's Grande Chirurgie. From post-classical Latin acorus, masculine, variant of classical Latin acoron, acorum (neuter) sweet flag or yellow flag from Hellenistic Greek ἄκορον yellow flag (Dioscorides), associated by ancient etymologists with ancient Greek κόρη pupil of the eye, as the plant was used medicinally to treat inflammation of the eye.