The Collaborative International Dictionary of English v.0.48:

Scarecrow \Scare"crow`\, n.
   1. Anything set up to frighten crows or other birds from
      cornfields; hence, anything terifying without danger.
      [1913 Webster]

            A scarecrow set to frighten fools away. --Dryden.
      [1913 Webster]

   2. A person clad in rags and tatters.
      [1913 Webster]

            No eye hath seen such scarecrows. I'll not march
            with them through Coventry, that's flat. --Shak.
      [1913 Webster]

   3. (Zool.) The black tern. [Prov. Eng.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

scarecrow
    n 1: an effigy in the shape of a man to frighten birds away from
         seeds [syn: scarecrow, straw man, strawman, bird-
         scarer, scarer]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "scarecrow":
   Dracula, Frankenstein, Wolf-man, baboon, bag, blemish, blot, bogey,
   bogeyman, bugaboo, bugbear, bust, carving, dog, doll, dolly, dummy,
   eyesore, fantoccini, fee-faw-fum, figure, figurehead, figurine,
   fright, frightener, gargoyle, ghost, ghoul, gingerbread man, hag,
   harridan, hobgoblin, holy terror, horror, incubus, lay figure,
   man of straw, manikin, mannequin, marionette, mess, model, monster,
   monstrosity, monument, nightmare, no beauty, ogre, ogress, phantom,
   portrait bust, puppet, revenant, scarebabe, scarer, sculpture,
   sight, snowman, specter, statuary, statue, statuette, succubus,
   tatterdemalion, teratism, terror, ugly duckling, vampire,
   wax figure, waxwork, werewolf, witch, wood carving