means Wood prepared for building, or trees considered as a source of such wood — and the warning yelled when a felled one is about to come crashing down.

from From Old English 'timber,' which originally meant a building or the material to build with, not just any old log. It traces back to a Proto-Germanic root (related to German 'Zimmer,' a room, and the verb 'zimmern,' to build with wood), and beyond that to a Proto-Indo-European root meaning 'to build' — a cousin of Greek 'demein,' to build, and 'domos,' a house. So timber and 'dome' and 'domestic' all share a distant ancestor obsessed with putting up shelter. The lumberjack's shouted 'Timber!' is a much later, distinctly woodsy use, but it fits: the word was always about the wood that makes things stand — or, in that case, fall.