whimsy
noun.
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
Synonym: notion, whim, whimsey
noun.
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