What is an idiom for an ultracrepidarian?

I make three suggestions (meddler and the more idiomatic backseat driver and busybody) but in confirming them have found others I did not know previously:

meddler - an officious annoying person who interferes with others an unwelcome person, persona non grata - a person who for some reason is not wanted or welcome

backseat driver - a meddler who insists on giving unwanted advice

busybody, nosey-parker, nosy-parker, quidnunc - a person who meddles in the affairs of others

buttinsky - a meddler who tends to butt in

kibitzer - (Yiddish) a meddler who offers unwanted advice to others

Free Dictionary

Other dictionaries are consistent with this conveniently compact group of descriptions.

A term denoting such a person, that (unlike ultracrepidarian) would be readily understood by most English speakers, is know-it-all. In British English, its variant know-all is also available. The term is inherently ironic: it means that the person does not, in fact, know very much, let alone 'all', but merely behaves as if he does.