Historically, graziers and farmers used this word to describe undesirable, aggressive weeds, brush, or briars that degraded grazing paddocks. It refers specifically to invasive vegetation that chokes out nutritious forage grasses and reduces the carrying capacity of the land. Clearing it usually required intensive manual labor, burning, or aggressive grazing by goats.
Heard on the job
We need to run the goats through the back forty to clear out all that filth before we seed it with clover.