Ultracrepidarian

An ultracrepidarian—from ultra- ("beyond") and crepidarian ("things related to shoes")—is a person considered to have ignored this advice and to be offering opinions they know nothing about.

The word is derived from a longer Latin phrase and refers to a story from Pliny the Elder

The phrase is recorded in Book 35 of Pliny the Elder's Natural History as ne supra crepidam sutor iudicaret[1] ("Let the cobbler not judge beyond the crepida") and ascribed to the Greek painter Apelles of Kos. Supposedly, Apelles would put new paintings on public display and hide behind them to hear and act on their reception.[2] On one occasion, a shoemaker (Latin sutor) noted that one of the crepides[a] in a painting had the wrong number of straps and was so delighted when he found the error corrected the next day that he started in on criticizing the legs.[2] Indignant, Apelles came from his hiding place and admonished him to confine his opinions to the shoes.[2] Pliny then states that since that time it had become proverbial.[2]