The Collaborative International Dictionary of English v.0.48:

Runt \Runt\, n. [Written also rant.] [Scot. runt an old cow,
   an old, withered woman, a hardened stem or stalk, the trunk
   of a tree; cf. D. rund a bullock, an ox or cow, G. rind. Cf.
   Rother, a.]
   1. (Zool.) Any animal which is unusually small, as compared
      with others of its kind; -- applied particularly to
      domestic animals.
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   2. (Zool.) A variety of domestic pigeon, related to the barb
      and carrier.
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   3. A dwarf; also, a mean, despicable, boorish person; -- used
      opprobriously.
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            Before I buy a bargain of such runts,
            I'll buy a college for bears, and live among 'em.
                                                  --Beau. & Fl.
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   4. The dead stump of a tree; also, the stem of a plant. [Obs.
      or Prov. Eng.] --Halliwell.
      [1913 Webster]

            Neither young poles nor old runts are durable.
                                                  --Holland.
      [1913 Webster]

WordNet (r) 3.0 (2006):

runt
    n 1: disparaging terms for small people [syn: runt, shrimp,
         peewee, half-pint]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "runt":
   Lilliputian, Tom Thumb, a nobody, a nothing, bantam, banty,
   brownie, button, chit, cipher, common man, diminutive, dummy,
   dwarf, elf, featherweight, figurehead, fingerling, gnome,
   homunculus, insignificancy, jackstraw, lightweight, lilliputian,
   little fellow, little guy, man of straw, manikin, mediocrity,
   midge, midget, mini, minikin, minnow, minny, mouse, nebbish,
   nobody one knows, nonentity, nubbin, obscurity, peewee, pip-squeak,
   pony, punk, pygmy, scrub, shrimp, slip, small fry, small potato,
   small potatoes, snip, snippet, squirt, squit, tit, wart, whiffet,
   whippersnapper, wisp