Whim (noun)

1. A sudden desire or change of mind, often without a good reason or explanation.
2. An impulse or fancy that is often fleeting or short-lived.

Origin:

From old english "hwim" meaning "a turning, change of mind".

Examples:

1. He left on a whim to travel the world.
2. She decided to dye her hair pink on a whim.
3. He quit his job on a whim and never looked back.
4. She bought a new car on a whim and regretted it later.
5. He proposed to her on a whim and she said yes.