The Collaborative International Dictionary
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.]
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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.
--Lady Murray. Old or refuse household stuff; things cumbrous, or bulky and useless, or of small value.
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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.
Wiktionary
n. storeroom in a house where odds and ends, especially furniture, can be stored
WordNet
n. a storeroom in a house where odds and ends can be stored (especially furniture)
Wikipedia
The large houses of the well-heeled of Britain commonly had much very old, well-built furniture in them, more than was to be used in every room at any given time. Every piece was made-to-order. When not needed, it was neither sold nor discarded by them. At least one out-of-the-way room was selected to store the pieces that were not in use. This was called the lumber room. Such is what is alluded to in the definition in the Oxford English Dictionary (OED), whose first reference is Richardson's novel Pamela and which is mentioned in a bit more detail in Daniel Pool's literary reference book of the 1990s, "What Jane Austen Ate and Charles Dickens Knew," among other literary reference works.
The phrase "lumber room" is found in British fiction at least during the 19th century (e.g., Arthur Conan Doyle's 1891 Sherlock Holmes short story " The Five Orange Pips"), and the use of the word lumber in this phrase is that found in many obsolescent turns of phrase heard in various English-speaking countries. Probably one of the most evocative references is the short story by "Saki" (H. H. Munro) called "The Lumber Room": "Often and often Nicholas had pictured to himself what the lumber-room might be like, that region that was so carefully sealed from youthful eyes and concerning which no questions were ever answered. It came up to his expectations. In the first place it was large and dimly lit, one high window opening on to the forbidden garden being its only source of illumination. In the second place it was a storehouse of unimagined treasures." In Dickens' A CHRISTMAS CAROL, The Lumber Room is one of several places Scrooge looks for intruders when he returns to his dismal, dark home after his 'melancholy meal in his melancholy tavern' on Christmas Eve.
The OED mentions in the verb "lumbering" that it first meant to obstruct with pieces of wood to make things from, and then shifted to general obstruction, hence furniture fit the later meaning.
Also referenced by J.R.R. Tolkien in Lord of the Rings. "His memory is like a lumber room. Thing wanted always buried." Found in book 1 of The Fellowship of the Ring.
Usage examples of "lumber room".
True, there was a lumber room with unfinished walls, but the other rooms were all finished, with interesting sloping ceilings and large windows bringing in plenty of light.
It's all rather something of a lumber room now, but much of this information was once quite important.
Pietro Massinello was a circus of one, allowed, like the others, to move his feast of dogs, cats, geese, and parakeets from the roof, where they lived in summer, to a basement lumber room in December, where they survived in a medley of barks, cackles, riots, and slumbers through the years.