Middle English: via Old French from Latin humor 'moisture', from humere (see humid). The original sense was 'bodily fluid' (surviving in aqueous humour and vitreous humour); it was used specifically for any of the cardinal humours (sense 3 of the noun), whence 'mental disposition' (thought to be caused by the relative proportions of the humours). This led, in the 16th century, to the senses 'mood' (sense 2 of the noun) and 'whim', hence to humour someone 'to indulge a person's whim'. sense 1 of the noun dates from the late 16th century.

• In the Middle Ages scientists and doctors believed that there were four main fluids in the body and that the relative proportions of these determined an individual's temperament. Blood gave a cheerful or sanguine disposition; phlegm made somebody stolidly calm or phlegmatic; choler or yellow bile gave a peevish and irascible, or choleric character; and melancholy or black bile caused depression. These substances were the four humours, or cardinal humours. From this notion humour acquired the sense ‘mental disposition’, then ‘state of mind, mood’ and ‘whim, fancy’ (hence to humour someone, ‘to indulge a person's whim’). The association with amusement arose in the late 17th century. The origin of humour directly refers to fluids—it derives from Latin humor ‘moisture’, from humere ‘to be moist’, source also of humid (Late Middle English).