To diminish or destroy the luster of; sully; dull: used of an alteration induced by the air, or by dust or dampness; also, in mineralogy, to change the natural color or luster of the surface of: said chiefly of the metallic minerals. See tarnish, n., 2.To give a pale or dim cast to, as to gold or silver, without either polishing or burnishing it.Figuratively, to diminish or destroy the purity of; cast a stain upon; sully: as, to tarnish reputation.Synonyms To dull, deface.To lose luster; become dim or dull: as, polished substances or gilding will tarnish in the course of time.n. A spot; a blot; the condition of being dulled or stained.n. In mineralogy, the change in luster or color of the surface of a mineral, particularly one of metallic luster: usually due to slight alteration, but also in some cases to the deposition of a very thin film of some foreign substance.n. A coating.