"Ultracrepidarian: someone who has no special knowledge of a subject but who expresses an opinion about it." (Cambridge Dictionary)
A neat word coined by William Hazlitt.
I found these uses of it:
1. "Like a conceited mechanic in a village alehouse, you would set down everyone who differs from you as an ignorant blockhead, and very fairly infer that anyone who is beneath yourself must be nothing. You have been well called an Ultra-Crepidarian critic." William Hazlitt, "Letter to William Gifford" (1819)
2. "In England excess in alcohol is responsible for only a very small proportion of cases of mental disease at all, in spite of ultracrepidarian views to the contrary." James R. Whitwell, letter to The English Review (March, 1929). p. 249.
3. "The Oklahoma Senate does not like what Secretary of Defense Charles E. Wilson said about the National Guard but it refused yesterday to call him 'ultracrepidarian.' A resolution had been introduced accusing Wilson of 'gross and unwarranted insult' .... Senator J. R. Hall, a Democrat, introduced an amendment which would have inserted 'ultracrepidarian.'" St. Louis Post-Dispatch (Jan. 31, 1957): 1.
4. "Now if Sino-Indo-European lexical comparison was just yet another drowsy backwater of the inveterate craze for ultracrepidarian transcontinental etymologizing, it would probably not even be justified to mention Ulenbrook's book in print." Wolfgang Behr, Review of Zum Alteurasischen. Eine Sprachvergleichung by Jan Ulenbrook, Oriens 36 (2001): 356-361, at 360.
5. "The FBI Hair Comparison Review does not seek to provide a precise and scientifically defensible estimate of the prevalence of ultracrepidarian testimony." David H. Kaye, "Ultracrepidarianism in Forensic Science: The Hair Evidence Debacle," Washington and Lee Law Review 72, no. 2 (2015): 230-257, at p. 247.