late 14c., rubarbe, medicinal root-stock of a plant native to China and Tibet, from Old French rubarbe and directly from Medieval Latin reubarbarbum, from Greek rha barbaron "foreign rhubarb," from rha "rhubarb," perhaps ultimately from a source akin to Persian rewend "rhubarb" (associated in Greek with Rha, ancient Scythian name of the River Volga) + barbaron, neuter of barbaros "foreign" (see barbarian).
It was long imported into Europe by way of Russia and became associated with that land. The European native species was so called by 1640s. The first vowel was altered in Medieval Latin by association with rheum. The restored -h- was occasional from Middle English but not established until late 18c.
The baseball slang meaning "loud squabble on the field" is from 1938, of unknown origin, said to have been first used by broadcaster Garry Schumacher. Perhaps it is connected with the use of rhubarb as a word repeated by stage actors to give the impression of hubbub or conversation (a stage effect attested from 1934).
evergreen shrub native to southern Europe and widely cultivated for its fragrance, late 14c., rose-marie, earlier rosmarine (c. 1300), from Latin rosmarinus, literally "dew of the sea" (compare French romarin), from ros "dew" + marinus "of the sea, maritime," from mare "sea, the sea, seawater" (from PIE root *mori- "body of water").
Perhaps it was so called because it grew near coasts. The form was altered in English by influence of unrelated rose and Mary.
Latin ros is from a PIE noun probably from *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)).