rosemary (n.) Look up rosemary at
late 14c., earlier rosmarine (c. 1300), from Latin rosmarinus, literally "dew of the sea" (compare 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" (source also of 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)).