E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
As good humour, ill or bad humour, etc. According to an ancient theory, there are four principal humours in the body: phlegm, blood, choler, and black bile. As any one of these predominates it determines the temper of the mind and body; hence the expressions sanguine, choleric, phlegmatic, and melancholic humours. A just balance made a good compound called good humour; a preponderance of any one of the four made a bad compound called an ill or evil humour. (See Ben Jonson: Every Man Out of His Humour (Prologue).