The Collaborative International Dictionary of English v.0.48:

Sow \Sow\, n. [OE. sowe, suwe, AS. sugu, akin to s[=u], D. zog,
   zeug, OHG. s[=u], G. sau, Icel. s[=y]r, Dan. so, Sw. sugga,
   so, L. sus. Gr. "y^s, sy^s, Zend. hu boar; probably from the
   root seen in Skr. s[=u] to beget, to bear; the animal being
   named in allusion to its fecundity. [root]294. Cf. Hyena,
   Soil to stain, Son, Swine.]
   1. (Zool.) The female of swine, or of the hog kind.
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   2. (Zool.) A sow bug.
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   3. (Metal.)
      (a) A channel or runner which receives the rows of molds
          in the pig bed.
      (b) The bar of metal which remains in such a runner.
      (c) A mass of solidified metal in a furnace hearth; a
          salamander.
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   4. (Mil.) A kind of covered shed, formerly used by besiegers
      in filling up and passing the ditch of a besieged place,
      sapping and mining the wall, or the like. --Craig.
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   Sow bread. (Bot.) See Cyclamen.

   Sow bug, or Sowbug (Zool.), any one of numerous species
      of terrestrial Isopoda belonging to Oniscus,
      Porcellio, and allied genera of the family Oniscidae.
      They feed chiefly on decaying vegetable substances.

   Sow thistle [AS. sugepistel] (Bot.), a composite plant
      (Sonchus oleraceus) said to be eaten by swine and some
      other animals.
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The Collaborative International Dictionary of English v.0.48:

Thistle \This"tle\, n. [OE. thistil, AS. [thorn]istel; akin to
   D. & G. distel, OHG. distila, distil, Icel. [thorn]istill,
   Sw. tistel, Dan. tidsel; of uncertain origin.] (Bot.)
   Any one of several prickly composite plants, especially those
   of the genera Cnicus, Craduus, and Onopordon. The name
   is often also applied to other prickly plants.
   [1913 Webster]

   Blessed thistle, Carduus benedictus, so named because it
      was formerly considered an antidote to the bite of
      venomous creatures.

   Bull thistle, Cnicus lanceolatus, the common large
      thistle of neglected pastures.

   Canada thistle, Cnicus arvensis, a native of Europe, but
      introduced into the United States from Canada.

   Cotton thistle, Onopordon Acanthium.

   Fuller's thistle, the teasel.

   Globe thistle, Melon thistle, etc. See under Globe,
      Melon, etc.

   Pine thistle, Atractylis gummifera, a native of the
      Mediterranean region. A vicid gum resin flows from the
      involucre.

   Scotch thistle, either the cotton thistle, or the musk
      thistle, or the spear thistle; -- all used national
      emblems of Scotland.

   Sow thistle, Sonchus oleraceus.

   Spear thistle. Same as Bull thistle.

   Star thistle, a species of Centaurea. See Centaurea.

   Torch thistle, a candelabra-shaped plant of the genus
      Cereus. See Cereus.

   Yellow thistle, Cincus horridulus.
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   Thistle bird (Zool.), the American goldfinch, or
      yellow-bird (Spinus tristis); -- so called on account of
      its feeding on the seeds of thistles. See Illust. under
      Goldfinch.

   Thistle butterfly (Zool.), a handsomely colored American
      butterfly (Vanessa cardui) whose larva feeds upon
      thistles; -- called also painted lady.

   Thistle cock (Zool.), the corn bunting (Emberiza
      militaria). [Prov. Eng.]

   Thistle crown, a gold coin of England of the reign of James
      I., worth four shillings.

   Thistle finch (Zool.), the goldfinch; -- so called from its
      fondness for thistle seeds. [Prov. Eng.]

   Thistle funnel, a funnel having a bulging body and flaring
      mouth.
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WordNet (r) 3.0 (2006):

Sonchus oleraceus
    n 1: annual Eurasian sow thistle with soft spiny leaves and
         rayed yellow flower heads [syn: milkweed, Sonchus
         oleraceus]