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late 14c., earlier rosmarine (c.1300), from Latin rosmarinus, literally "dew of the sea" (cf. French romarin), from ros "dew" + marinus (see marine (adj.)). Perhaps so called because it grew near coasts. Form altered in English by influence of rose and Mary.
Latin ros is from PIE *ers- "to be wet" (cf. Lithuanian rasa, Old Church Slavonic rosa "dew," Sanskrit rasah "sap, juice, fluid, essence," Hittite arszi "flows," and perhaps also Rha, Scythian name of the River Volga (see rhubarb)).