zapatero, a tus zapatos

Etymology
An anecdote attributed to Apelles; When a cobbler commented on his mistakes in painting a shoe, Apelles made the corrections that very night; the next morning the cobbler noticed the changes, and proud of his effect on the artist's work began to criticize how Apelles portrayed the leg—whereupon Apelles emerged from his hiding-place to state: Ne sutor ultra crepidam—"Let the shoemaker venture no further."

Phrase

 * 1)  cobbler, keep to your last used to criticize someone who opines about a subject without knowing it