c.1412, "proportioned mixture of elements," from L.
temperamentum "proper mixture," from
temperare "to mix" (see
temper). In medieval theory, it meant a combination of qualities (hot, cold, moist, dry) that determined the nature of an organism; this was extended to a combination of the four humors (sanguine, choleric, phlegmatic, and melancholic) that made up a person's characteristic disposition. General sense of "habit of mind, natural disposition" is from 1821.
Temperamental "of or pertaining to temperament" is from 1646; in the sense of "moody" it is recorded from 1907.