In the existentialist philosophy of Sartre: the self-deception practised by a person who wishes to avoid absolute responsibility for his or her own actions by not recognizing that human beings have free will and that no relationship is genuinely mutual. Also (in extended use): bad faith, disingenuousness.
1940s. From French mauvaise foi, lit. ‘bad faith’ from mauvaise, feminine of mauvais bad + foi faith.