hard as the nether millstone

(as) hard as the nether millstone

Tough or harsh and unlikely to submit. The phrase is Biblical in origin. That guy is as hard as the nether millstone—he'll never give you what you want in the negotiations.
See also: hard, millstone, nether
Farlex Dictionary of Idioms.

hard as the nether millstone

callous and unyielding.
The nether millstone is the lower of the two millstones by which corn is ground. The phrase alludes to Job 41:24: ‘His heart is as firm as a stone, and as hard as a piece of the nether millstone’.
See also: hard, millstone, nether
Farlex Partner Idioms Dictionary
See also:
  • (as) hard as the nether millstone
  • millstone
  • nether
  • corn in Egypt
  • apple of
  • apple of eye
  • apple of my eye
  • apple of one's eye
  • get thee behind me
  • as you sow, so shall you reap
References in classic literature
But if you plod along, for half an hour, over this soft, easy carpet - giving way at every step, yielding the more the harder you press, - you'll find it rather wearisome work, and be glad enough to come to a bit of good, firm rock, that won't budge an inch whether you stand, walk, or stamp upon it; and, though it be hard as the nether millstone, you'll find it the easier footing after all.'
The Tenant of Wildfell Hall