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]