Since the dawn of the industrial age, anything that is unspectacular yet functional has been called “run of the mill.”
When a raw product is to be mechanically processed, whether through a gristmill or the mill of a mine, it emerges in bulk before the different sizes and qualities have been separated by value.
Worth can’t be determined until further refining and so everything looks the same, and that’s why anything ordinary is called run of the mill.
Something ordinary said to be “run of the mill”.
