British folk tales and songs had it that all millers were deceitful, adding chalk to the flour, overcharging and selling under weight. It’s the same caricature that says all policemen have big feet, all tailors are timid, and all farmers say “oh arr!”
Our song has a miller questioning his sons so that his mill can be left to the biggest cheat. It was first printed in the 1600s and is found all over the English speaking world in almost identical versions, though with different tunes. This one was collected in the 1960s in the USA.