Run of the mill
The ordinary, basic article, with no decoration or augmentation.
There are many 'run of the...' phrases that have been used to denote ordinariness of some commodity or other. The meaning of all of these phrases is broadly the same, that is, they refer to products that come direct from the mill in an ungraded state and may contain some imperfections. They differ of course from phrases like 'having the run of the mill', which would denote the freedom to roam around the mill.
Early examples of this are 'run of the kiln', 'run of the mine' etc. These refer to 'runs', that is, periods of active use of the industrial process in question; for example, The 1909 Century Dictionary Supplement defined these as:
"Run of the kiln, bricks of all kinds and qualities just as they happen to come from the kiln."
"Run of the mine, coal just as it comes from the mine, large and small sizes and all qualities together."
Run of the mill is a little earlier than those and is American in origin. The mill in question was a weaving mill and the articles first called 'run of the mill' were clothes. An early citation of that comes from an advert by Cook, Taylor & Co. of Lowell, Massachusetts in The Lowell Daily Sun, December 1895:
"Seconds and the run of the mill, but for all wearing purposes just the same as firsts at twice the price. Fleeced Jersey Vests in white or Ecru, 2 for 25c."
In more recent years the 'run of the...' franchise has extended to just about anything; for example, in November 1988 The New York Times printed a review of the film Rent-a-Cop, starring Liza Minnelli, in which it described it as an failed attempt to "mate a romantic comedy with a run-of-the-studio shoot-'em-down".
See other phrases that were coined in the USA.