A millstone is a large, heavy, circular stone used to grind grain into flour. For thousands of years, mills used pairs of millstones: one stone stayed still while another rotated on top of it, crushing wheat, corn, or other grains between them. Some millstones weighed over a thousand pounds. Water wheels or windmills provided the power to turn these massive stones, and the grinding surface had grooves that helped move the grain toward the edges as it was crushed into flour.

Because millstones were so heavy and difficult to move, the word took on a second meaning: a burden that weighs someone down and makes progress nearly impossible. When someone describes a problem as a millstone around their neck, they mean it's dragging them down and preventing them from moving forward. A student might feel that unfinished homework from weeks ago has become a millstone, making it harder to focus on current assignments. A business might discover that an old debt has become a millstone, preventing growth and success.