Hard as the Nether Millstone
Unfeeling, obdurate. The lower or “nether” of the two millstones is firmly fixed and very hard; the upper stone revolves round it on a shaft, and the corn, running down a tube inserted in the upper stone, is ground by the motion of the upper stone round the lower one. Of course, the upper wheel is made to revolve by some power acting on it, as wind, water, or some other mechanical force.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894