In the English language, black sheep is an idiom used to describe an odd or disreputable member of a group, especially within a family. The term stems from sheep whose fleece is colored black rather than the more common white; these sheep stand out in the flock and their wool was traditionally considered less valuable.
The term originated from the occasional black sheep which are born into a flock of white sheep. Black wool was considered commercially undesirable because it could not be dyed. In 18th and 19th century England, the black color of the sheep was seen as the mark of the devil. In modern usage, the expression has lost some of its negative connotations, though the term is usually given to the member of a group who has certain characteristics or lack thereof deemed undesirable by that group.