Millstone is a noun that refers to a heavy burden.

Traditionally, a millstone is a heavy stone used to crush something. This is the origin of our word of the day, but in more recent use, millstone is generally applied metaphorically.

For example: Kari used to think of that piano in her living room as a millstone, something that would be a real pain to deal with when it was time to move. But since she’s learned to play the thing, she doesn’t think of it as a burden at all.