Actually, it was an English term. When the colonists first came here,
in their speech (according to Eric Sloane author of lots of books on
the subject) they considered 'timber' to be good wood. The word
'lumber' was derogatory, meaning 'trash wood' or something like that.
Since they were really interested in clearing land for crops & the
entire countryside was forest, they started calling most wood 'lumber'.
I think that was reinforced in the early 1800's when they turned
entire forests into charcoal to feed the iron works. They used to use
what we would consider prime wood for road beds! sigh They didn't
know how good they had it.

Anyway, that's Eric Sloane's explanation & it made sense of a sort.
When you consider that the US was first known to the world for
exporting White Pine & some other woods that were highly prized, it
doesn't make as much sense. When you realize that they used wood for
most everything from dishes to roads, it makes less sense. When you
think about how they razed the first growth forest though...

Jim