'run-of-the-mill' originates from English, specifically the phrase 'run of the mill', where 'run' meant an output or batch produced by a mill.

'run-of-the-mill' changed from the literal expression 'run of the mill' (used in manufacturing to mean a mill's regular output) and eventually became the idiomatic adjective 'run-of-the-mill' meaning ordinary or undistinguished.

Initially, it meant 'the output produced by a mill (a regular batch)', but over time it evolved into the current meaning of 'ordinary; not special'.