There are lots of things we say which originated in milling. I covered "show your mettle" here https://gnatbottomedtowers.blogspot.com/search?q=show+your+mettle . " Rule of thumb" refers to when the miller took a pinch of flour and rubbed it between his thumb and forefinger to check the grind. If it was still too coarse it would need to be reground. "The daily grind" meant the everyday standard grind and "keep your nose to the grindstone" occurred when the grindstones were set wrongly which could make them hot and cook the flour. It was not unknown for the flour to get so hot it would burst into flames and it was often the cause of the wooden mills burning down. There are many more.