means The shrubs, saplings, and low tangled vegetation growing beneath the larger trees of a forest.

from A transparent English compound: "under" (beneath) plus "brush" in its sense of cut or scrubby vegetation. That "brush" came into English from Old French "broce" / "brosse," meaning brushwood or scrub — the same root that gave us the bristly cleaning brush, since early brushes were literally bundled twigs. "Underbrush" itself is an American coinage, surfacing in the 18th century to name the dense low growth a settler had to hack through under the canopy.