Is there an idiom for a person who is in the habit of giving advice or suggestions on matters he or she knows nothing about? I know there is this word ultracrepidarian for such people but I'm looking for an idiom.A phrase would do too.
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
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.