John Lightfoot on Luke 17:2

[That a millstone were hanged about his neck.] There is mention among the Talmudic authors, concerning an ass-mill, and it is distinguished from a hand-mill. "Whoso hireth a house of his neighbour, he may build an ass-mill, but not a hand-mill."

To have a millstone hanged about his neck was a common proverb. "Samuel saith, It is a tradition, that a man may marry, and after that apply himself to the study of the law. But R. Jochanan saith, No. Shall he addict himself to the study of the law with a millstone about his neck?"

Suidas tells us, When they drowned any in the sea, they hung stones about their necks. And quotes that of Aristophanes:

Lifting him up, I'll plunge him to the deep,

A stone hung at his neck.