mill ‧ stone /ˈmɪlstəʊn $ -stoʊn/ BrE AmE noun [countable]
1 . one of the two large circular stones that crush grain into flour in a ↑ mill
2 . a millstone round/around sb’s neck something that causes a lot of problems for someone, and that they cannot get rid of:
Inflation is still a millstone round the neck of British businesses.