E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Let not the cobbler overstep his last (Ne sutor ultra crepidam). Let no one presume to interfere in matters of which he is ignorant. The tale goes that a cobbler detected a fault in the shoe-latchet of one of Apelles paintings, and the artist rectified the fault. The cobbler, thinking himself very wise, next ventured to criticise the legs; but Apelles answered, Keep to your tradeyou understand about shoes, but not about anatomy.