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. Its present figurative use was first recorded in a history of the Quakers (c. 1720).
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.