millstone (millstones plural )
1 n-count A millstone is a large, flat, round stone which is one of a pair of stones used to grind grain into flour.
2 If you describe something as a millstone or a millstone around your neck, you mean that it is a very unpleasant problem or responsibility that you cannot escape from.
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a millstone/a millstone around one's neck phrase
(disapproval)
For today's politicians, the treaty is becoming a millstone...