A dye made by mixing two different dyes; especially one used to dye the two materials of a union cloth, typically cotton (cellulose) and wool or silk (protein).
Origin
Mid 19th century.
Definition of union dye in US English:
union dye
noun
A dye made by mixing two different dyes; especially one used to dye the two materials of a union cloth, typically cotton (cellulose) and wool or silk (protein).