whimsy

/ˈwɪmzi/
noun
  1. A playful or fanciful idea, quality, or object; something unusual or amusing that is not serious.
    • The architect added a touch of whimsy to the building with a spiral slide instead of stairs.
    • The garden was full of whimsy, with brightly painted gnomes and a tiny castle.
    • Her writing has a gentle whimsy that makes even ordinary events feel magical.
  2. A sudden, odd, or unpredictable idea or desire; a whim.
    • The trip was pure whimsy — they picked a destination by closing their eyes and pointing at a map.
    • He bought the old camera on a whimsy, not knowing if it worked.
    • On a whimsy, she decided to dye her hair purple.
Antonyms
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