The Collaborative International Dictionary of English v.0.48:

Weathercock \Weath"er*cock`\, n.
   1. A vane, or weather vane; -- so called because originally
      often in the figure of a cock, turning on the top of a
      spire with the wind, and showing its direction. "As a
      wedercok that turneth his face with every wind."
      --Chaucer.
      [1913 Webster]

            Noisy weathercocks rattled and sang of mutation.
                                                  --Longfellow.
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   2. Hence, any thing or person that turns easily and
      frequently; one who veers with every change of current
      opinion; a fickle, inconstant person.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Weathercock \Weath"er*cock`\, v. t.
   To supply with a weathercock; to serve as a weathercock for.
   [1913 Webster]

         Whose blazing wyvern weathercock the spire. --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

weathercock
    n 1: weathervane with a vane in the form of a rooster