self-appointed
adjective disapproving uk/ˌself.əˈpɔɪn.tɪd/ us/ˌself.əˈpɔɪn.t̬ɪd/
自命的;自封的
behaving as if you have responsibility or authority but without having been chosen by other people
The newspaper has become the self-appointed guardian of public morals. 这家报纸自命为公共道德的维护者。