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 Naturalis Historia [XXXV, 36, 85-86 (Loeb IX, 323-325)] where he records that a shoemaker (sutor) had approached the painter Apelles of Kos 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 most likely coined the term Ultracrepidarian, first used publicly in a ferocious letter to William Gifford, the editor of The Quarterly Review:
A related English proverb is "A cobbler should stick to his last" (a last being the wooden pattern used to mould the shoe).