a.

1. 想入非非的，异想天开的
   a whimsical notion

   离题的想法

   At that moment, a whimsical idea occurred to me: why don't I propose to her from outer space?

   那个时候，我突然有个异想天开的念头：我为什么不从外太空向她求婚？

2. 怪诞的，古怪的
   a whimsical man

   古怪的家伙

3. 反复无常的

adj.

1. playfully quaint or fanciful, especially in an appealing and amusing way.
2. acting or behaving in a capricious manner.