Synonyms for Whimsey (same or very similar meaning)
WordNet sense 1 (an odd or fanciful or capricious idea):
notion, whim, whimsyWordNet sense 2 (the trait of acting unpredictably and more from whim or caprice than from reason or judgment):
arbitrariness, capriciousness, flightiness, whimsicalityFrom the ODE community, based on WordNetadd/edit