whimsy

Concise Oxford English Dictionary © 2008 Oxford University Press:
whimsy /ˈwɪmzi/ (also whimsey)

▶noun (pl. whimsies or whimseys)

• 1 playfully quaint or fanciful behaviour or humour.

  ■ a thing that is fanciful or odd.

• 2 a whim.

– origin C17 (in the sense ‘caprice’): prob. based on whim-wham.