1The rear part of a house, or a subsidiary building situated to the rear of the main premises of a house, especially one used for a specific purpose, as a wash house, storage space, etc. Chiefly regional in later use (now rare).
2North American A small building or room containing a toilet, especially one which is behind or outside a main building.
Perhaps also compare earlier "in (the) backhouse ditch", which may allude to a cesspit.
Mid 16th century. From back- + house.