sleep with

   to copulate with

   Perhaps the commonest use, normally of extramarital copulation by either sex, or both, and now standard English:

    One couldn't accept a fur coat without sleeping with a man. (G. Greene, 1932)

   A sleeping dictionary is a native-speaking mistress from whom you hope to learn the language:

    East African (European) officers as a whole maintained a veiy much stricter code in the matter of sleeping with African women... sometimes referred to as 'sleeping dictionaries', from their obvious advantages as language instructors. (C. Allen, 1975)

   A sleeping partner, with whom you regularly copulate, puns on the part-owner who plays no active part in the running of the business:

    ... the services of a Somali girl-friend or sleeping partner, (ibid.)

   Also rarely as sleepy time girl, who can be a mistress or a prostitute:

    Seems like the bint was one of his sleepy time girls. (Chandler, 1953)