The Collaborative International Dictionary of English v.0.48:

Lumber \Lum"ber\, n. [Prob. fr. Lombard, the Lombards being the
   money lenders and pawnbrokers of the Middle Ages. A lumber
   room was, according to Trench, originally a Lombard room, or
   room where the Lombard pawnbroker stored his pledges. See
   Lombard.]
   1. A pawnbroker's shop, or room for storing articles put in
      pawn; hence, a pledge, or pawn. [Obs.]
      [1913 Webster]

            They put all the little plate they had in the
            lumber, which is pawning it, till the ships came.
                                                  --Lady Murray.
      [1913 Webster]

   2. Old or refuse household stuff; things cumbrous, or bulky
      and useless, or of small value.
      [1913 Webster]

   3. Timber sawed or split into the form of beams, joists,
      boards, planks, staves, hoops, etc.; esp., that which is
      smaller than heavy timber. [U.S.]
      [1913 Webster]

   Lumber kiln, a room in which timber or lumber is dried by
      artificial heat. [U.S.]

   Lumber room, a room in which unused furniture or other
      lumber is kept. [U.S.]

   Lumber wagon, a heavy rough wagon, without springs, used
      for general farmwork, etc.

   dimensional lumber, lumber, usually of pine, which is sold
      as beams or planks having a specified nominal
      cross-section, usually in inches, such a two-by-four,
      two-by-six, four-by-four, etc.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

lumber room
    n 1: a storeroom in a house where odds and ends can be stored
         (especially furniture)