A lumber room is a room, most often in the attic of a house, used for storing unused possessions such as

furniture Furniture refers to objects intended to support various human activities such as seating (e.g., Stool (seat), stools, chairs, and sofas), eating (table (furniture), tables), storing items, working, and sleeping (e.g., beds and hammocks). Furnitur ...

and other items the household has been "lumbered with", or accumulated over time. "Lumber" meaning to give something of little use or worth to the recipient that cannot be easily gotten rid of.

References

Rooms {{room-stub