The Collaborative International Dictionary of English v.0.48:

Back \Back\, a.
   1. Being at the back or in the rear; distant; remote; as, the
      back door; back settlements.
      [1913 Webster]

   2. Being in arrear; overdue; as, back rent.
      [1913 Webster]

   3. Moving or operating backward; as, back action.
      [1913 Webster]

   Back blocks, Australian pastoral country which is remote
      from the seacoast or from a river. [Webster 1913 Suppl.]


   Back charges, charges brought forward after an account has
      been made up.

   Back filling (Arch.), the mass of materials used in filling
      up the space between two walls, or between the inner and
      outer faces of a wall, or upon the haunches of an arch or
      vault.

   Back pressure. (Steam Engine) See under Pressure.

   Back rest, a guide attached to the slide rest of a lathe,
      and placed in contact with the work, to steady it in
      turning.

   Back slang, a kind of slang in which every word is written
      or pronounced backwards; as, nam for man.

   Back stairs, stairs in the back part of a house; private
      stairs. Also used adjectively. See Back stairs,
      Backstairs, and Backstair, in the Vocabulary.

   Back step (Mil.), the retrograde movement of a man or body
      of men, without changing front.

   Back stream, a current running against the main current of
      a stream; an eddy.

   To take the back track, to retrace one's steps; to retreat.
      [Colloq.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Backstairs \Back"stairs`\, Back stairs \Back" stairs`\
   (b[a^]k"st[^a]rz`) n.
   Stairs in the back part of a house, as distinguished from the
   front stairs; a second staircase at the rear of a building;
   hence, a private or indirect way.
   [1913 Webster + WordNet 1.5] Backstairs

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "back stairs":
   back door, back road, back street, back way, bolt-hole, by-lane,
   bypass, bypath, byroad, bystreet, byway, companion, companionway,
   covert way, detour, escalier, escalier derobe, escape hatch,
   escape route, fire escape, flight of steps, incline, landing,
   landing stage, perron, ramp, roundabout way, secret exit,
   secret passage, secret staircase, side door, side road,
   side street, spiral staircase, staircase, stairs, stairway,
   stepping-stones, steps, stile, treads and risers, underground,
   underground railroad, underground route