The generic term used after the Norman Conquest to refer to land which had formerly been cultivated and settled but had fallen into neglect. Depopulated and discarded, the land reverted to wilderness -- first overgrown with grass, then with scrub and finally with trees. In some cases the waste was a deliberate policy, either of the overlord of the land, or of the tenants who simply moved away. In others settlements simply failed, and no-one bothered to keep them up. Some, of course, became waste when William I's army was engaged in suppressing rebellions.
The phrase became more commonplace when it was used as a generic term in the Domesday Book, when it served as a rationale for reduced or lost income, eiother at the time of King Edward or subsequently.