means A playful, fanciful quality or a sudden capricious notion — the impulse to do something light and odd just because it pleases you.

from From 'whimsy-whamsy,' a 17th-century reduplicative coinage — the kind of bouncy rhyming doubling English loves (like 'hocus-pocus' or 'mishmash'). It likely springs from 'whim-wham,' a trinket or trifling fancy of uncertain root; the playful sound seems to have done as much work as any meaning. By the late 1600s 'whim' had budded off on its own, and 'whimsy' kept the giddy spin of its doubled origins.