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.
      [1913 Webster]

   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

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "weathercock":
   April showers, Proteus, Vicar of Bray, anemograph, anemometer,
   anemometrograph, anemoscope, barometer, chameleon, cloud shapes,
   cock, feeler, formalist, kaleidoscope, mercury, moon,
   pilot balloon, probe, quicksilver, random sample, rolling stone,
   sample, shifting sands, sound, sounder, straw vote, temporizer,
   the weather, timepleaser, timeserver, trial balloon, trimmer, vane,
   water, weather vane, wheel of fortune, whirligig, wind cone,
   wind indicator, wind sock, wind vane, wind-speed indicator