either of a pair of circular stones between which grain or another substance is ground, as in a mill.

anything that grinds or crushes.

any heavy mental or emotional burden (often used in the phrase a millstone around one's neck ).

noun

accountability , affliction , albatross * , anxiety , ball and chain , blame , charge , concern , cross , deadweight , difficulty , encumbrance , grievance , hardship , hindrance , load , mental weight , misfortune , onus , punishment , responsibility , strain , stress , task , thorn in one’s side , trial , trouble , worry , tax , weight , burden