A cobbler is one who makes his living by repairing boots and shoes. The term is also used for that superior craftsman, a shoemaker, who uses a wooden or metal model called a ‘last’ for shaping the footwear he produces. But however clever he may be at his trade, he should not try to give advice on other matters.
The Roman writer, Pliny the Elder, tells this story about the painter Apelles: ‘He was in the habit of hanging his pictures where they could be seen by the passers-by, and listening to their comments. One day a shoemaker criticised the shoes in a certain picture, and found next day that they had been repainted. Proud of his success as a critic, he began to find fault with a thigh of the picture, when Apelles called out from behind the canvas, “Shoemaker, don’t go above your last!” ‘ (Stevenson’s Book of Quotations.)
The proverb applies to anyone who tries to teach someone else his business. A plumber does not welcome the advice of an electrician; and an electrician does not welcome the advice of a plumber.