thioindigo (iou)

thioindigo noun. Pl. -os. E20.
[from THIO- + INDIGO.]
A red vat dye in which the two imino groups of indigotin are replaced by sulphur atoms. Also, = thioindigoid noun.
thioindigoid noun & adjective (a) noun any of a class of mainly orange, red, and violet vat dyes that are substituted derivatives of thioindigo, used esp. in textile printing; (b) adjective of the nature of a thioindigoid: M20.