E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
means an undue mixture. In medicine a distemper arises from the redundancy of certain secretions or morbid humours. The distemper in dogs is an undue quantity of secretions manifested by a running from the eyes and nose. (Latin, dis-tempero, to mix amiss.)
Applied to painting, the word is from another source, the French détremper (to soak in water), because the paints, instead of being mixed with oil, are mixed with a vehicle (as yolk of eggs or glue) soluble in water.