distemper (v.)

late 14c., distemperen, "to disturb, upset the proper balance of," from Old French destemprer and directly from Medieval Latin distemperare "vex, make ill," literally "upset the proper balance (of bodily humors)," from dis- "un-, not" (see dis-) + Latin temperare "mingle in the proper proportion" (see temper (v.)). Related: Distempered.

"unbalanced or unnatural temper," 1550s, from distemper (v.). Middle English expressed the idea by distempering, distemperure.  From 1640s as "disease of the body, malady, indisposition;" specifically in reference to a wasting disease of young dogs by 1747, later extended to other animals.