black sheep

Short definition from Wiktionary

Noun

1. 1
   A nonconformist; an unusual or unconventional person, who may be rejected or disdained by others for this reason.
2. 2
   A person who is not wholesome, honest, or trustworthy.
3. 3
   A worker who refuses to strike; a scab.
4. 4
   Used other than figuratively or idiomatically: see black, sheep.