The American Heritage dictionary defines a 'Freudian slip' as: "A verbal mistake that is thought to reveal an unconscious belief, thought, or emotion."
An anonymous aphorism goes, "A Freudian slip is when you say one thing, and you mean your mother."
Named after Dr. Sigmund Freud, an early psychoanalyst who postulated that many unconscious thoughts (especially those surrounding early, formative, and sexual experiences) manifest themselves in a person's outward behaviors.