Ul·tra·crep·i·dar·i·an

A person who expresses opinions on matters outside the scope of their knowledge or expertise.

An ultracrepidarian is someone who gives opinions and advice on matters beyond their knowledge or competence — essentially, a person who speaks authoritatively about things they know nothing about.

It comes from the Latin phrase ultra crepidam ("beyond the sandal"), rooted in a story about the ancient Greek painter Apelles. When a cobbler criticized one of his paintings, Apelles accepted the critique of the sandal — within the cobbler's expertise — but when the cobbler moved on to criticizing the legs, Apelles reportedly shot back: ne sutor ultra crepidam ("cobbler, not above the sandal"). The word was coined in English by William Hazlitt in 1819.

It's a useful concept — the idea that confidence and competence often come apart, and that people routinely venture opinions well outside their actual knowledge. In that sense, it's a close cousin to the Dunning-Kruger effect, though ultracrepidarian is far more satisfying to say.