|a person who is regarded as a disgrace or failure by his family or peer group|
A person who is considered a disgrace to a particular group, usually a family: “Uncle Jack, who was imprisoned for forgery, is the black sheep of the family.”
The least reputable member of a group; a disgrace. For example, Uncle Fritz was the black sheep of the family; we always thought he emigrated to Argentina to avoid jail. This metaphor is based on the idea that black sheep were less valuable than white ones because it was more difficult to dye their wool different colors. Also, in the 16th century, their color was considered the devil's mark. By the 18th century the term was widely used as it is today, for the odd member of a group.