Someone who strongly disagrees with another person's fair opinion/criticism. It came from is a baseball term, "foul ball".
John Doe: Justin Bieber acts like a arrogant fool all the time. He sucks.
Jane Doe: No, he isn't. He is a talented singer.
John Doe: Did you just cry foul?
Jane Doe: No, he isn't. He is a talented singer.
John Doe: Did you just cry foul?
by DPST TMPST February 17, 2014