From German. Meaning "knowing better". And that's exactly what it refers to, like the more common "smart ass" or "wise ass", or even "smart elick" (smart aleck). A besserwisser - or wise ass - is a person, often intellectual, or at least pseudo-intellectual, who thinks he or she has more general knowledge about things than most others. It can be correcting language mistakes in grammar and spelling, or it can be within a specific topic like everything from politics, geography and baseball to history and science - anything you can imagine!
- You know that "bananarama" dude at the Madonna forum? Jeesuz, he's so annoying! Such a besserwisser! Whenever I make the tiniest spelling mistake, he's there immediately to correct me and make me look everything but good. And whenever there's a discussion, he always claims he is the one with the correct answer. Every single time - it never fails! Even though I've caught him in misleading knowledge at least a handful of times!