The Collaborative International Dictionary of English v.0.48:

Grape \Grape\, n. [OF. grape, crape, bunch or cluster of grapes,
   F. grappe, akin to F. grappin grapnel, hook; fr. OHG. chrapfo
   hook, G. krapfen, akin to E. cramp. The sense seems to have
   come from the idea of clutching. Cf. Agraffe, Cramp,
   Grapnel, Grapple.]
   1. (Bot.) A well-known edible berry growing in pendent
      clusters or bunches on the grapevine. The berries are
      smooth-skinned, have a juicy pulp, and are cultivated in
      great quantities for table use and for making wine and
      raisins.
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   2. (Bot.) The plant which bears this fruit; the grapevine.
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   3. (Man.) A mangy tumor on the leg of a horse.
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   4. (Mil.) Grapeshot.
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   Grape borer. (Zool.) See Vine borer.

   Grape curculio (Zool.), a minute black weevil (Craponius
      in[ae]qualis) which in the larval state eats the interior
      of grapes.

   Grape flower, or

   Grape hyacinth (Bot.), a liliaceous plant (Muscari
      racemosum) with small blue globular flowers in a dense
      raceme.

   Grape fungus (Bot.), a fungus (Oidium Tuckeri) on
      grapevines; vine mildew.

   Grape hopper (Zool.), a small yellow and red hemipterous
      insect, often very injurious to the leaves of the
      grapevine.

   Grape moth (Zool.), a small moth (Eudemis botrana), which
      in the larval state eats the interior of grapes, and often
      binds them together with silk.

   Grape of a cannon, the cascabel or knob at the breech.

   Grape sugar. See Glucose.

   Grape worm (Zool.), the larva of the grape moth.

   Sour grapes, things which persons affect to despise because
      they can not possess them; -- in allusion to [AE]sop's
      fable of the fox and the grapes.
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The Collaborative International Dictionary of English v.0.48:

Sour \Sour\, a. [Compar. Sourer; superl. Sourest.] [OE.
   sour, sur, AS. s?r; akin to D. zuur, G. sauer, OHG. s?r,
   Icel. s?rr, Sw. sur, Dan. suur, Lith. suras salt, Russ.
   surovui harsh, rough. Cf. Sorrel, the plant.]
   1. Having an acid or sharp, biting taste, like vinegar, and
      the juices of most unripe fruits; acid; tart.
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            All sour things, as vinegar, provoke appetite.
                                                  --Bacon.
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   2. Changed, as by keeping, so as to be acid, rancid, or
      musty, turned.
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   3. Disagreeable; unpleasant; hence; cross; crabbed; peevish;
      morose; as, a man of a sour temper; a sour reply. "A sour
      countenance." --Swift.
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            He was a scholar . . .
            Lofty and sour to them that loved him not,
            But to those men that sought him sweet as summer.
                                                  --Shak.
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   4. Afflictive; painful. "Sour adversity." --Shak.
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   5. Cold and unproductive; as, sour land; a sour marsh.
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   Sour dock (Bot.), sorrel.

   Sour gourd (Bot.), the gourdlike fruit Adansonia
      Gregorii, and Adansonia digitata; also, either of the
      trees bearing this fruit. See Adansonia.

   Sour grapes. See under Grape.

   Sour gum (Bot.) See Turelo.

   Sour plum (Bot.), the edible acid fruit of an Australian
      tree (Owenia venosa); also, the tree itself, which
      furnished a hard reddish wood used by wheelwrights.
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   Syn: Acid; sharp; tart; acetous; acetose; harsh; acrimonious;
        crabbed; currish; peevish.
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WordNet (r) 3.0 (2006):

sour grapes
    n 1: disparagement of something that is unattainable

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "sour grapes":
   acid, acidulant, belittling, bread-and-butter pickle, chokecherry,
   comedown, contempt, crab apple, decrial, depreciation, derogation,
   detraction, dill pickle, disapproval, discrediting, disgrace,
   disparagement, faint praise, green apple, indignity, knocking,
   lemon, lime, lukewarm support, minimizing, pickle, putting down,
   slighting, sour, sour balls, sour cream, sour pickle, sourdough,
   verjuice, vinegar, yogurt