Chapter Three: What is all "lumber" according to Jerome, the writer.

The "lumber" would be things that are unnecessary..... clothing, servants, friends who aren't really friends, and things that are meant to impress.

How many people, on that voyage, load up the boat till it is ever in danger of swamping with a store of foolish things which they think essential to the pleasure and comfort of the trip, but which are really only useless lumber. How they pile the poor little craft mast-high with fine clothes and big houses; with useless servants, and a host of swell friends that do not care twopence for them, and that they do not care three ha’pence for; with expensive entertainments that nobody enjoys, with formalities and fashions, with pretence and ostentation, and with — oh, heaviest, maddest lumber of all! — the dread of what will my neighbour think, with luxuries that only cloy, with pleasures that bore, with empty show that, like the criminal’s iron crown of yore, makes to bleed and swoon the aching head that wears it!