from The Century Dictionary and Cyclopedia
- n. A pupil in a boarding-school who has many privileges not granted to the ordinary pupils.
Sorry, no etymologies found.
Though there is a tradition in the school that, a short time after the founding of the establishment, the late lamented husband of Mrs. Smith, who was fond of equestrian exercise, kept a horse -- which a parlor-boarder, niece of Mrs. Smith, was allowed to ride -- hence the provision in the circular.
So it happened that, about half an hour afterward, Sara struck the entire school-room of pupils dumb with amazement, by making her appearance in a costume such as she had never worn since the change of fortune whereby she ceased to be a show-pupil and a parlor-boarder.