millstone

Short definition from Wiktionary

Noun

1. 1
   A large round stone used for grinding grain.
2. 2
   A coarse-grained sandstone used for making such stones; millstone grit.
3. 3
   Ellipsis of millstone round one's neck, a heavy responsibility that is difficult to bear (referring to Matthew 18:6 in the Bible).

Proper noun

1. 1
   A nuclear power station near Waterford, Connecticut, USA.
2. 2
   An unincorporated community in Mercer County, New Jersey, United States.
3. 3
   A township in Monmouth County, New Jersey.
4. 4
   A borough in Somerset County, New Jersey.
5. 5
   A tributary of the Raritan River in New Jersey; in full, the Millstone River.