Millstone (noun)

1. A heavy stone, typically circular, used for grinding grain.
2. A task or burden that is heavy, dull, and never-ending.

Origin:

Middle english mille ston, from old english mylen stān, from mylen, mill + stān, stone.

Examples:

1. He was the millstone around her neck, the burden she carried through life.
2. The new regulations were a millstone for small businesses, slowing them down and making it harder for them to succeed.
3. The old millstones at the museum were still covered in the flour they had once ground.
4. Her job as a caregiver had become a millstone, wearing her down both physically and emotionally.
5. The millstone was the central component of the grain-grinding process, and was powered by horses, mules, or water.