v.
  - lose or cause to lose lustre, especially as a result of exposure to air or moisture.
 - make less valuable or respected:
 they make up negative stories to tarnish my image.
 
n.
 loss of brightness. 
▸a film or stain formed on an exposed surface of a mineral or metal.
 
  Derivative
  
  Etymology
  ME: from Fr. terniss-, lengthened stem of ternir, from terne ‘dark, dull’.