In addition the fluids which tempered the tides of the blood in Galen's ancient model of human makeup, the word also came to mean a passing fancy or sentiment, or whimsy.

"humours - In medieval physiology, four liquids in the human body affecting behavior. Each humour was associated with one of the four elements of nature. In a balanced personality, no humour predominated. When a humour did predominate, it caused a particular personality. ...".

I suspect, but do not know for sure, that the usages meaning "whimsey" and "funny" are descendants of the Athenian usage. The Ayurveda school of medicine which developed originally around the time of Buddha uses a similar model, and the word may come from the Sanskrit.

"AMBITION is like choler; which is an humor that maketh men active, earnest, full of alacrity, and stirring, if it be not stopped. "
(Francis Bacon)

Origin:
1300–50; ME (h)umour < AF < L (h)ūmōr- (s. of (h)ūmor) moisture, fluid (medical L: body fluid), equiv. to (h)ūm(ēre) to be wet (see humid ) + -ōr- -or 1