Source : Webster's Revised Unabridged Dictionary (1913)

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.

            All sour things, as vinegar, provoke appetite.
                                                  --Bacon.

   2. Changed, as by keeping, so as to be acid, rancid, or
      musty, turned.

   3. Disagreeable; unpleasant; hence; cross; crabbed; peevish;
      morose; as, a man of a sour temper; a sour reply. ``A sour
      countenance.'' --Swift.

            He was a scholar . . . Lofty and sour to them that
            loved him not, But to those men that sought him
            sweet as summer.                      --Shak.

   4. Afflictive; painful. ``Sour adversity.'' --Shak.

   5. Cold and unproductive; as, sour land; a sour marsh.

   {Sour dock} (Bot.), sorrel.

   {Sour gourd} (Bot.), the gourdlike fruit {Adansonia
      Gregorii}, and {A. 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.

   Syn: Acid; sharp; tart; acetous; acetose; harsh; acrimonious;
        crabbed; currish; peevish.

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.

   2. (Bot.) The plant which bears this fruit; the grapevine.

   3. (Man.) A mangy tumor on the leg of a horse.

   4. (Mil.) Grapeshot.

   {Grape borer}. (Zo["o]l.) See {Vine borer}.

   {Grape curculio} (Zo["o]l.), 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} (Zo["o]l.), a small yellow and red hemipterous
      insect, often very injurious to the leaves of the
      grapevine.

   {Grape moth} (Zo["o]l.), 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} (Zo["o]l.), 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.

Source : WordNet®

sour grapes
     n : disparagement of something that is unattainable