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 humors (humor (sense 3 of the noun)), whence ‘mental disposition’ (thought to be caused by the relative proportions of the humors). This led, in the 16th century, to the senses ‘mood’ (humor (sense 2 of the noun)) and ‘whim’, hence to humour someone ‘to indulge a person's whim’. humor (sense 1 of the noun) dates from the late 16th century