John Lightfoot on Matthew 18:6

[It were better for him that a millstone were hanged about his neck, &c.] It is good for him, in Talmudic language.

A millstone seems to be said in distinction from those very small mills wherewith they were wont to grind the spices that were either to be applied to the wound of circumcision, or to be added to the delights of the sabbath. Hence the Gloss of R. Solomon upon Jeremiah 25:10; "The sound of mills and the light of the candle": "The sound of mills (saith he), wherewith spices were ground and bruised for the healing of circumcision."

That Christ here speaks of a kind of death, perhaps nowhere, certainly never used among the Jews; he does it either to aggravate the thing, or in allusion to drowning in the Dead sea, in which one cannot be drowned without some weight hung to him: and in which to drown any thing, by a common manner of speech, implied to devote to rejection, hatred, and execration; which we have observed elsewhere.