D defs.my
Entry 1 sense Webster, 1913

Millstone

/mĭl'-stōnˌ/ · Mill·stone · IPA /ˈmɪlˌstoʊn/
01 n. One of two circular stones used for grinding grain or other substance in a mill{1}.
  1. 1.
    One of two circular stones used for grinding grain or other substance in a mill{1}.
    “No man shall take the nether or the upper millstone to pledge.” — Deut. xxiv. 6.
Phrases & compounds
Millstone girt — a hard and coarse, gritty sandstone, dividing the Carboniferous from the Subcarboniferous strata. See Farewell rock, under Farewell, a., and Chart of Geology.
To see into a millstone — to see into or through a difficult matter.