Run of the Mill

[TariqE]

I know what it means but where does it come from?

[Jules001]

It comes from the fact that provided there was a steady flow of water coming from the mill-pond, the mill would run smoothly and normally

[New Forester]

Presumably it's the ordinary flour produced all the time by the mill, as opposed to something extra fine or otherwise unusual.

[Kingaroo]

I recently visited a historic grist mill with docents demonstrating the milling process. Their information corroborates New Forester's.

[Katboy]

Bear in mind that a mill is (was) not solely a place where grain was ground.

Most early factories, before automation, were referred to as 'mills', especially those which produced textiles.

So, 'run of the mill' might also refer to a rougher grade of cloth that was mass-produced by a mill in its normal run, as opposed to a finer grade for special occasions.

I realise that this is virtually the same answer as for flour....