Slang for someone who gives opinions unasked

In British English you might use:

know-it-all - someone who behaves as if they have all the answers to something, and know more than everybody else, usually giving these answers without being asked.

busybody - from the two words busy meaning engaged in an activity and body meaning person - somebody who interferes in others' affairs.

nosy parker - slightly archaic, nosy meaning interested in things which do not concern them, and Parker being a proper noun.

There aren't actually many precise words for this, but people that do it are said to butt in or mouth off.

Noun: kibitzer ki-bit-su(r)

(Yiddish) a meddler who offers unwanted advice to others

Derived forms: kibitzers

Type of: meddler

-- WordWeb on line

And the verb is kibitz:

Verb: kibitz ki-bits

Make unwanted and intrusive comments

"Don't kibitz: he's sensitive";

kibbitz [N. Amer]

Derived forms: kibitzed, kibitzing, kibitzes

Type of: comment, notice, point out, remark

buttinsky or buttinski, according to Merriam-Webster:

a person given to butting in --- a troublesome meddler.

Example of buttinsky in a sentence: there's always some buttinsky who has to tell you there's a better way to do whatever you're doing.

Although the word originated in the US, here are several examples from the OED of use by British writers:

1933 D. L. Sayers Murder must Advertise iv. 69, I never..met with such a bunch of buttinskis... Nothing is sacred to you.

1960 P. G. Wodehouse Jeeves in Offing v. 50 It is never pleasant for a man of sensibility to find himself regarded as a buttinski and a trailing arbutus

For origin, the OED says: