tarnish

Short definition from Wiktionary

Noun

1. 1
   Oxidation or discoloration, especially of a decorative metal exposed to air.

Verb

1. 1
   To oxidize or discolor due to oxidation.
2. 2
   To compromise, damage, soil, or sully.
3. 3
   To lose its lustre or attraction; to become dull.
4. 4
   To use a sign, image, expression, etc. sufficiently close to a trademarked one that it brings disrepute to it.