"to bite gently; eat by gnawing off small bits," c. 1500, not found in Middle English; perhaps from Low German nibbeln "to nibble, gnaw," related to Middle Low German nibbelen, Middle Dutch knibbelen "to gnaw," source of Dutch knibbelen "to cavail, squabble." Related: Nibbled; nibbling.
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).