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,
   Rhaponticine.]
   1. (Bot.) The name of several large perennial herbs of the
      genus Rheum and order Polygonaceae.
      [1913 Webster]

   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.
      [1913 Webster]

   3. (Med.) The root of several species of Rheum, used much
      as a cathartic medicine.
      [1913 Webster]

   Monk's rhubarb. (Bot.) See under Monk.

   Turkey rhubarb (Med.), the roots of Rheum Emodi.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rhubarb
    n 1: long pinkish sour leafstalks usually eaten cooked and
         sweetened [syn: pieplant, rhubarb]
    2: plants having long green or reddish acidic leafstalks growing
       in basal clumps; stems (and only the stems) are edible when
       cooked; leaves are poisonous [syn: rhubarb, rhubarb
       plant]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "rhubarb":
   Bedlam let loose, Irish potato, Kraut, altercation, apologetics,
   apologia, apology, argument, argumentation, aubergine, barney,
   beans, bedlam, beef, bicker, bickering, blast, bobbery, brawl,
   brouhaha, cabbage, casuistry, charivari, chirm, clamor, clangor,
   clap, clatter, commotion, contention, controversy, defense, din,
   discord, disputation, dispute, donnybrook, drunken brawl, dustup,
   eggplant, falling-out, flap, flyting, fracas, free-for-all, greens,
   hassle, hell broke loose, howl, hubbub, hue and cry, hullabaloo,
   jangle, knock-down-and-drag-out, legumes, litigation, logomachy,
   loud noise, love apple, mad apple, noise, noise and shouting,
   outcry, pandemonium, paper war, passage of arms, pieplant, polemic,
   polemics, potato, potherbs, produce, racket, rattle, roar, row,
   ruckus, ruction, rumble, rumpus, run-in, scrap, set-to, shindy,
   shivaree, spud, tater, thunder, thunderclap, tintamarre, tomato,
   tumult, uproar, vegetables, verbal engagement, war of words,
   white potato, wrangling