The Collaborative International Dictionary of English v.0.48:
Rhubarb \Rhu"barb\, n. [F. rhubarbe, OF. rubarbe, rheubarbe,
reubarbare, reobarbe, LL. rheubarbarum for rheum barbarum,
Gr. ??? (and ??) rhubarb, from the river Rha (the Volga) on
whose banks it grew. Originally, therefore, it was the
barbarian plant from the Rha. Cf. Barbarous,
1. (Bot.) The name of several large perennial herbs of the
genus Rheum and order Polygonaceae.
2. The large and fleshy leafstalks of Rheum Rhaponticum and
other species of the same genus. They are pleasantly acid,
and are used in cookery. Called also pieplant.
3. (Med.) The root of several species of Rheum, used much
as a cathartic medicine.
Monk's rhubarb. (Bot.) See under Monk.
Turkey rhubarb (Med.), the roots of Rheum Emodi.