mid-14c., "fluid or juice of an animal or plant," from Old North French humour (Old French humor; 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," from PIE *wegw- "wet."
In ancient and medieval physiology, "any of the four body fluids" (blood, phlegm, choler, and melancholy or black bile) whose relative proportions were thought to determine state of mind. This led to a sense of "mood, temporary state of mind" (first recorded 1520s); the sense of "amusing quality, funniness" is first recorded 1680s, probably via sense of "whim, caprice" (1560s), which also produced the verb sense of "indulge," first attested 1580s. "The pronunciation of the initial h is only of recent date, and is sometimes omitted ...." [OED] For types of humor, see the useful table below, from H.W. Fowler ["Modern English Usage," 1926].