A millstone around one's neck
What's the meaning of the phrase 'A millstone round his neck'?
A heavy and inescapable burden or responsibility.
What's the origin of the phrase 'A millstone round his neck'?
This phrase is biblical in origin and its origins are found in Matthew 18:6 in The Wycliffe Bible, circa 1395:
It spedith to hym that a mylnestoon of assis be hangid in his necke and be drenchid in to the depnesse of the see.
You don't need to be a scholar of Middle English to get the gist of that. The biblical punishment of having a heavy weight put round one's neck and being cast into the sea was a retribution for 'offending the little ones who believe in me'. Presumably, swimming would then be pretty much of a vertical affair.
The expression has been in continuous use in English since at least the 14th century. These days a weighty burden is often referred to as the shortened form 'millstone'.
Note: the millstones in the picture are some of the many such that were left lying around in the Peak District surface mines when the grinding trade in nearby Sheffield went into decline.