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), 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) and ‘whim’, hence to humour someone ‘to indulge a person's whim’. humor dates from the late 16th century.