2historical Diarrhoea that is pale or mucoid; an instance of this; a disease causing such diarrhoea, specifically tropical sprue.
Originally: a white substance, consisting chiefly of potassium carbonate, used as a flux in the smelting and assaying of metals. Now also: any of various other white substances used as a flux (e.g. in welding, soldering, or brazing), often containing salts of boron or fluorine.
Early 16th century; earliest use found in The Grete Herball. From white + flux.