The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with run of the mill
Ordinary, average, as in There's nothing special about these singers—they're just run of the mill. This expression alludes to fabrics coming directly from a mill without having been sorted or inspected for quality. It has survived such similar phrases as run of the mine and run of the kiln, for the products of mines and kilns.
[ Late 1800s