a blot on your escutcheon

a blot on (one's) escutcheon

A stain or mark against one's reputation or that of one's family. An escutcheon was a heraldic shield that bore a family's coat of arms, and thus serves as a metaphor for one's honor. I was at the top of my class in college, but that narcotics charge will be a blot on my escutcheon for years to come.
See also: blot, escutcheon, on

a blot on your escutcheon

or

a blot on the escutcheon

BRITISH, OLD-FASHIONED
If there is a blot on your escutcheon, you have damaged your reputation by doing something wrong. For the leaders, this is probably a blip rather than a blot on the escutcheon. Note: An escutcheon is a shield, especially a heraldic shield displaying a coat of arms.
See also: blot, escutcheon, on