Bay rum \Bay" rum"\ A fragrant liquid, used for cosmetic and medicinal purposes. Note: The original bay rum, from the West Indies, is prepared, it is believed, by distillation from the leaves of the bayberry ({Myrcia acris}). The bay rum of the Pharmacop[oe]ia (spirit of myrcia) is prepared from oil of myrcia (bayberry), oil of orange peel, oil of pimento, alcohol, and water.