The word “mill” originally referred to a wide variety of buildings that were equipped with machines that were used to grind wheat and other grains into flour. As new machines were invented, some of them were considered specialized mills such as the windmill and sawmill. When the textile processing machine was invented, it too was referred to as “the textile mill,” or just “the mill.” The textile mill was used to generate yards of fabric. This fabric was named “run of the mill” because it had just come straight out of the textile milling machine. It would then be taken to other machines where it was to be cut, graded for quality, or dyed into different colors. There was no way of knowing the quality of the “run of the mill” fabric until it had undergone further processing. So, the phrase “run of the mill” took on the meaning “product of undefined quality.” 

From there, the phrase gained wide usage, and it’s now used to refer to things that are ordinary, average, or basic, and are not outstanding in any way of rarity or quality.