What is a negative word to describe people that love showing off their knowledge?

A "Clavin" is dated but still funny. Cliff Clavin was one of the main characters on one of the most popular sitcoms of all time in the US - Cheers. He is the EXACT person that you describe in the most extreme way.

Terms I would use to describe Clavin would be:

1. Windbag
2. Bigmouth
3. Blowhard

Clavin Quote #1:

Well ya see, Norm, it's like this. A herd of buffalo can only move as fast as the slowest buffalo. And when the herd is hunted, it is the slowest and weakest ones at the back that are killed first. This natural selection is good for the herd as a whole, because the general speed and health of the whole group keeps improving by the regular killing of the weakest members. In much the same way, the human brain can only operate as fast as the slowest brain cells. Excessive intake of alcohol, as we know, kills brain cells. But naturally, it attacks the slowest and weakest brain cells first. In this way, regular consumption of beer eliminates the weaker brain cells, making the brain a faster and more efficient machine. That's why you always feel smarter after a few beers.

Clavin Quote #2:

CLIFF CLAVIN - You know Rebecca, I don't know why you're so concerned about your figure. You know, back in the Renaissance times, full figured women were revered.

NORM PETERSON- Get out.

CLIFF CLAVIN - It's true. Yeah, yeah, artists would only paint big, voluptuous women. In fact, that's how they got rid of a lot of their old paint.

Clavin Quote #3

You know, there's a lot of misunderstanding about rats. The rodendus vermikitis as they're called in Latin. It turns out our long tailed friend wasn't after all responsible for the dreaded bubonic plague as alleged through history. Yes, sir. It was caused by an animal called the bubon. That's right, and the threat by the way is still with us. So if anyone does see a bubon, contact your local authorities.

Clavin Quote #4

Norm, it's a little known fact that the word Florida comes from the language of the Okie Canokie Indians and it means, literally, place where the old people come to sweat.

Clavin Quote #5

You see Sammy, topless waitresses, scientific fact, they can deliver drinks faster than their clothed counterparts.

Know-it-all is a good noun that in my opinion is somewhat slangy: http://dictionary.reference.com/browse/know-it-all. For me, this has the connotation of not always being correct.

Show-off is another noun that comes to mind: http://dictionary.reference.com/browse/show-off

The word to use is Pedant.

The interesting part is that it has two pronunciations:

1. /'pɛdənt/ (pedd-unt)
2. /'pidənt/ (peed-unt)

So, whichever pronunciation you use in accusing someone,
if they truly are a pedant, they will correct your pronunciation.

If you don't mind being slangy, you might call such a person a smart-arse:-

someone who is always trying to seem more clever than other people in a way that is annoying: I don't want some smart-arse from the city telling me how to manage my farm.

(American smart-ass; in either case don't expect them to like it.)