lumber (n.) Look up lumber at Dictionary.com
"timber sawn into rough planks," 1660s, American English (Massachusetts), earlier "disused bit of furniture; heavy, useless objects" (1550s), probably from lumber (v.), perhaps influenced by Lombard, from the Italian immigrants famous as pawnbrokers and money-lenders in England (see Lombard). Lumbar, Lumbard were old alternative forms of Lombard in English. The evolution of sense then would be because a lumber-house ("pawn shop") naturally accumulates odds and ends of furniture.
Live Lumber; soldiers or passengers on board a ship are so called by the sailors.


LUMBER HOUSE. A house appropriated by thieves for the reception of their stolen property. ["Dictionary of Buckish Slang, University Wit, and Pickpocket Eloquence," London, 1811]
lumber (v.) Look up lumber at Dictionary.com
"to move clumsily," c.1300, lomere, probably from a Scandinavian source (cf. dialectal Swedish loma "move slowly, walk heavily," Old Norse lami "lame"), ultimately cognate with lame (adj.). Related: Lumbered; lumbering.