whimsical

determined by chance or impulse or whim rather than by necessity or reason

Full of, or characterized by, whims; actuated by a whim; having peculiar notions; queer; strange; freakish.

from Latin 'vis' meaning force or impulse

“Romfrey, with a whimsical shrewd cast of the eye at Beauchamp, who stood alert not to be foiled, arrow-like in look and readiness to repeat his home-shot.”

— George Meredith, Complete Project Gutenberg Works of George Meredith

“The term antimatter was first used by Arthur Schuster in two rather whimsical letters to Nature in 1898, in which he coined the term.”