The Collaborative International Dictionary of English v.0.48:

Whimsey \Whim"sey\, Whimsy \Whimsy\, n.; pl. Whimseysor
   Whimsies. [See Whim.]
   [1913 Webster]
   1. A whim; a freak; a capricious notion, a fanciful or odd
      conceit. "The whimsies of poets and painters." --Ray.
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            Men's folly, whimsies, and inconstancy. --Swift.
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            Mistaking the whimseys of a feverish brain for the
            calm revelation of truth.             --Bancroft.
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   2. (Mining) A whim.
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The Collaborative International Dictionary of English v.0.48:

Whimsey \Whim"sey\, v. t.
   To fill with whimseys, or whims; to make fantastic; to craze.
   [R.]
   [1913 Webster]

         To have a man's brain whimsied with his wealth. --J.
                                                  Fletcher.
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The Collaborative International Dictionary of English v.0.48:

Whim \Whim\, n. [Cf. Icel. hwima to wander with the eyes, vim
   giddiness, Norw. kvima to whisk or flutter about, to trifle,
   Dan. vimse to skip, whisk, jump from one thing to another,
   dial. Sw. hvimsa to be unsteady, dizzy, W. chwimio to move
   briskly.]
   [1913 Webster]
   1. A sudden turn or start of the mind; a temporary
      eccentricity; a freak; a fancy; a capricious notion; a
      humor; a caprice.
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            Let every man enjoy his whim.         --Churchill.
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   2. (Mining) A large capstan or vertical drum turned by horse
      power or steam power, for raising ore or water, etc., from
      mines, or for other purposes; -- called also whim gin,
      and whimsey.
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   Whim gin (Mining), a whim. See Whim, 2.

   Whim shaft (Mining), a shaft through which ore, water,
      etc., is raised from a mine by means of a whim.
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   Syn: Freak; caprice; whimsey; fancy.

   Usage: Whim, Freak, Caprice. Freak denotes an
          impulsive, inconsiderate change of mind, as by a child
          or a lunatic. Whim is a mental eccentricity due to
          peculiar processes or habits of thought. Caprice is
          closely allied in meaning to freak, but implies more
          definitely a quality of willfulness or wantonness.
          [1913 Webster]

WordNet (r) 3.0 (2006):

whimsey
    n 1: an odd or fanciful or capricious idea; "the theatrical
         notion of disguise is associated with disaster in his
         stories"; "he had a whimsy about flying to the moon";
         "whimsy can be humorous to someone with time to enjoy it"
         [syn: notion, whim, whimsy, whimsey]
    2: the trait of acting unpredictably and more from whim or
       caprice than from reason or judgment; "I despair at the
       flightiness and whimsicality of my memory" [syn:
       flightiness, arbitrariness, whimsicality, whimsy,
       whimsey, capriciousness]