A slang word used to refer to a friend who is a backstabber, not a good friend, or just a really sneaky, slimy type of friend or acquaintance. In other words, a fake friend. The word originated from the 1993 movie Menace 2 Society, in which the lead character has a fake friend named Chauncy who sets him up and snitches.
Guy A: "Yo man, I don't care if she was your girl, she says I taste better fool."
Guy B. "oh hell naw, you acting like a real Chauncy right now!"