mid-14c., "fluid or juice of an animal or plant," from Old North French humour "liquid, dampness; (medical) humor" (Old French humor, umor; Modern French humeur), from Latin umor "body fluid" (also humor, by false association with humus "earth"); related to umere "be wet, moist," and to uvescere "become wet" (see humid).

In old medicine, "any of the four body fluids" (blood, phlegm, choler, and melancholy or black bile) whose relative proportions were thought to determine physical condition and state of mind. This led to a sense of "mood, temporary state of mind" (first recorded 1520s); the sense of "amusing quality, funniness, jocular turn of mind" is first recorded 1680s, probably via sense of "whim, caprice" as determined by state of mind (1560s), which also produced the verb sense of "indulge (someone's) fancy or disposition." Modern French has them as doublets: humeur "disposition, mood, whim;" humour "humor." "The pronunciation of the initial h is only of recent date, and is sometimes omitted ..." [OED].

For aid in distinguishing the various devices that tend to be grouped under "humor," this guide, from Henry W. Fowler ["Modern English Usage," 1926] may be of use:

HUMOR: motive/aim : discovery; province : human nature; method/means : observation; audience : the sympathetic

WIT: motive/aim : throwing light; province : words & ideas; method/means : surprise; audience : the intelligent

SATIRE: motive/aim : amendment; province : morals & manners; method/means : accentuation; audience : the self-satisfied

SARCASM: motive/aim : inflicting pain; province : faults & foibles; method/means : inversion; audience : victim & bystander

INVECTIVE: motive/aim : discredit; province : misconduct; method/means : direct statement; audience : the public

IRONY: motive/aim : exclusiveness; province : statement of facts; method/means : mystification; audience : an inner circle

CYNICISM: motive/aim : self-justification; province : morals; method/means : exposure of nakedness; audience : the respectable