From Robert Jordan's Wheel of Time fantasy series. Used when referring to young novices or Accepted (in training to become Aes Sedai) who are very close friends. Does not neccessarily have a sexual connotation, but refers to young women who share a room or a bed talking late at night about their goals, dreams, experiences: i.e. slumber party.
While pillow-friends as Accepted, Moiraine and Siuan
plan to put mice in Elaida's bed, but they are caught and switched.