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.
      [1913 Webster]

            All sour things, as vinegar, provoke appetite.
                                                  --Bacon.
      [1913 Webster]

   2. Changed, as by keeping, so as to be acid, rancid, or
      musty, turned.
      [1913 Webster]

   3. Disagreeable; unpleasant; hence; cross; crabbed; peevish;
      morose; as, a man of a sour temper; a sour reply. "A sour
      countenance." --Swift.
      [1913 Webster]

            He was a scholar . . .
            Lofty and sour to them that loved him not,
            But to those men that sought him sweet as summer.
                                                  --Shak.
      [1913 Webster]

   4. Afflictive; painful. "Sour adversity." --Shak.
      [1913 Webster]

   5. Cold and unproductive; as, sour land; a sour marsh.
      [1913 Webster]

   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.
      [1913 Webster]

   Syn: Acid; sharp; tart; acetous; acetose; harsh; acrimonious;
        crabbed; currish; peevish.
        [1913 Webster]

WordNet (r) 3.0 (2006):

sour dock
    n 1: European sorrel with large slightly acidic sagittate leaves
         grown throughout north temperate zone for salad and spring
         greens [syn: sour dock, garden sorrel, Rumex acetosa]