tarnished

Concise Oxford English Dictionary © 2008 Oxford University Press:
tarnish/ˈtɑːnɪʃ/

▶verb

• 1 lose or cause to lose lustre, especially as a result of exposure to air or moisture.

• 2 make less valuable or respected: they make up negative stories to tarnish my image.

▶noun loss of brightness.

■ a film or stain formed on an exposed surface of a mineral or metal.

– derivatives
tarnishable adjective.

– origin ME: from Fr. terniss-, lengthened stem of ternir, from terne ‘dark, dull’.