a door or gate at the back of a building or yard; refers to the opposite of 前门 qián​mén

unfair or dishonest ways of getting things done through personal connections or bribery; under-the-table dealings

(computing) a hidden entry point in a software system or computer that bypasses normal security

(slang) the anus

(literary) a person of humble origin or a low-ranking family

(figurative) a means of retreat; a way out of a difficult situation