Millstones are circular stones that have been used since time immemorial to crush grain. Since they were supposed to crush grain, millstones had to be extremely heavy stones. This implied heaviness, by extension, has become their figurative use.

The dictionary definitions for millstone are as follows:
1. Either of a pair of circular stones between which grain or another substance is ground, as in a mill (noun).
2. Anything that grinds or crushes (noun).
3. Any heavy mental or emotional burden (often used in the phrase a millstone around one’s neck ) (noun).

Idiom based on MILLSTONE:

Millstone around one’s neck: A heavy burden, as in Julie finds Grandma, who is crabby, a millstone around her neck . The literal hanging of a millstone about the neck is mentioned as a punishment in the New Testament (Matthew 18:6), causing the miscreant to be drowned. A look at the picture below should illustrate the idea even better.

Usage Examples for MILLSTONE:
1. Left to fight the world alone, with the millstone of her shame around her neck, she had sunk ever lower and lower. -Three Men In A Boat (To Say Nothing Of The Dog) by Jerome, Jerome K.
2. Narmonee took my meaning in an instant, and running out of the house, returned the next moment with a huge rough mass of rock as big as a millstone, and indicated to me that that was exactly the thing I wanted. –Typee by Melville, Herman