(verb)
to destroy the shine/luster of something or to cause it to become dull or discolored, typically due to exposure to air, dirt, or moisture, or to be subjected to this destruction;
e.g. The humidity will tarnish these metallic objects.
(noun)
a film or stain on the surface of a metal, caused by the chemical alteration of the metal due to air, dirt, or moisture, that results in the metal losing its shine/luster or becoming dull or discolored;
e.g. She used a sponge to scrub the tarnish off the silver dish.
(noun)
harm or damage that is done to something or someone in an attempt to make it/them less respected or valued;
e.g. He never recovered from the tarnish on his character, and has struggled to find a job ever since.