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
1. A pawnbroker's shop, or room for storing articles put in
pawn; hence, a pledge, or pawn. [Obs.]
They put all the little plate they had in the
lumber, which is pawning it, till the ships came.
2. Old or refuse household stuff; things cumbrous, or bulky
and useless, or of small value.
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.]
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]