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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The Collaborative International Dictionary of English v.0.48:

Tupelo \Tu"pe*lo\, n. [Tupelo, or tupebo, the native American
   Indian name.] (Bot.)
   A North American tree (Nyssa multiflora) of the Dogwood
   family, having brilliant, glossy foliage and acid red
   berries. The wood is crossgrained and very difficult to
   split. Called also black gum, sour gum, and pepperidge.
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   Largo tupelo, or Tupelo gum (Bot.), an American tree
      (Nyssa uniflora) with softer wood than the tupelo.

   Sour tupelo (Bot.), the Ogeechee lime.
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WordNet (r) 3.0 (2006):

sour gum
    n 1: columnar tree of eastern North America having horizontal
         limbs and small leaves that emerge late in spring and have
         brilliant color in early fall [syn: sour gum, black
         gum, pepperidge, Nyssa sylvatica]