Sutor, ne ultra crepidam is a Latin expression meaning literally "Shoemaker, not above the sandal", used to warn people off passing judgment beyond their expertise.

Its origin is set down in Pliny the Elder's

Pliny the Elder

Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...

 Naturalis Historia

Naturalis Historia

The Natural History is an encyclopedia published circa AD 77–79 by Pliny the Elder. It is one of the largest single works to have survived from the Roman Empire to the modern day and purports to cover the entire field of ancient knowledge, based on the best authorities available to Pliny...

[XXXV, 36, 85-86 (Loeb IX, 323-325)] where he records that a shoemaker (sutor) had approached the painter Apelles

Apelles

Apelles of Kos was a renowned painter of ancient Greece. Pliny the Elder, to whom we owe much of our knowledge of this artist rated him superior to preceding and subsequent artists...

 of Kos

Kos

Kos or Cos is a Greek island in the south Sporades group of the Dodecanese, next to the Gulf of Gökova/Cos. It measures by , and is from the coast of Bodrum, Turkey and the ancient region of Caria. Administratively the island forms a separate municipality within the Kos peripheral unit, which is...

 to point out a defect in the artist's rendition of a sandal (crepida from Greek krepis), which Apelles duly corrected. Encouraged by this, the shoemaker then began to enlarge on other defects he considered present in the painting, at which point Apelles advised him that ne supra crepidam sutor iudicaret (a shoemaker should not judge above the sandal), which advice, Pliny observed, had become a proverbial saying.

The English essayist William Hazlitt

William Hazlitt

William Hazlitt was an English writer, remembered for his humanistic essays and literary criticism, and as a grammarian and philosopher. He is now considered one of the great critics and essayists of the English language, placed in the company of Samuel Johnson and George Orwell. Yet his work is...

 most likely coined the term Ultracrepidarian

Ultracrepidarianism

Ultracrepidarianism is the habit of giving opinions and advice on matters outside of one's knowledge.The term ultracrepidarian was first publicly recorded in 1819 by the essayist William Hazlitt in a letter to William Gifford, the editor of the Quarterly Review: "You have been well called an...

, first used publicly in a ferocious letter to William Gifford

William Gifford

William Gifford was an English critic, editor and poet, famous as a satirist and controversialist.-Life:Gifford was born in Ashburton, Devonshire to Edward Gifford and Elizabeth Cain. His father, a glazier and house painter, had run away as a youth with vagabond Bampfylde Moore Carew, and he...

, the editor of The Quarterly Review:

1819 HAZLITT Letter to W. Gifford Wks. 1902 I. 368 You have been well called an Ultra-Crepidarian critic. (Oxford English Dictionary 2nd ed.)

A related English proverb is "A cobbler should stick to his last" (a last being the wooden pattern used to mould the shoe).