Bad Apple Meaning

(idiomatic) A person who is not wholesome, honest, or trustworthy, especially one who has an adverse influence on others.

Example: 1956, "Echoing Ring (book review of Bang the Drum Slowly by Mark Harris)," Time, 19 Mar.,
  There is one bad apple, and that is Katie, the beautiful prostitute with whom Catcher Bruce is in love. Unlike the cliche harlot of fiction, she is as short of compassion as Bruce is of IQ.
2003, Steven Greenhouse, "Unions See Politics in New Disclosure Rules," New York Times, 5 Oct. (retrieved 30 July 2008),
  While most union leaders are people of integrity, there are still bad apples.