capricious; fanciful; lighthearted; arising in the mind suddenly and randomly
I just had this whimsical idea: why don't we go down to the movie theater and see whatever film just happens to be showing?
Whimsical looks like we + him + musical. We have selected him for his extraordinary, and impulsive performance. A musician never performs because of necessity or reason. This is their passion which forced him to give impulsive performance. Whimsical = WHISTLE. when you see a gal, you whistle. That is behave with impulse rather than reasoning.