The modern sense evolved in the late 17th century. Earlier meanings were chiefly related to the idea of "humours" as fluids in the body controlling physical, mental, and emotional dispositions. In Shakespeare, the word usually means "capricious, moody". When Le Beau describes Duke Frederick as "humorous" (As You Like It I.ii.255) he is not thinking about his joke-telling ability; nor was Menenius renowned for his laughs, though describing himself as a "humorous patrician" (Coriolanus II.i.44). People were described as humorous, as were their temperaments (Jaques's "sadness", As You Like It IV.i.18) and associated emotional noises (Berowne's sighs, Love's Labour's Lost III.i.172). Just once, something non-human is said to be humorous, and here the medieval sense emerges, meaning "humid" or "damp": Romeo consorts with "the humorous night" (Romeo and Juliet II.i.31).
David Crystal is the author, with Ben Crystal, of Shakespeare's Words, published by Penguin