whimsey

[See Whim.]

1. A whim; a freak; a capricious notion, a fanciful or odd conceit. «The whimsies of poets and painters.» Ray.

Men's folly, whimsies, and inconstancy.

Mistaking the whimseys of a feverish brain for the calm revelation of truth.

2. (Mining) A whim.

Whim·sey
v. t.

To fill with whimseys, or whims; to make fantastic; to craze. [R.]

To have a man's brain whimsied with his wealth.

The noun has 2 meanings:

Meaning #1: a whimsical idea
  Synonyms: notion, whim, whimsy

Meaning #2: the trait of acting more from whim or caprice than from reason or judgment
  Synonyms: flightiness, arbitrariness, whimsicality, whimsy, capriciousness