late 14c., "proportioned mixture of elements," from Latin temperamentum "proper mixture, a mixing in due proportion," from temperare "to mix" (see temper (v.)). In medieval theory, it meant a combination of qualities (hot, cold, moist, dry) that determined the nature of an organism; thus also "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.
