Used to emphasize that what was just said is merely a guess or assumption. (Sometimes used sarcastically.) If I had to take a guess, I'd say that the fan belt is loose. But I'm not a mechanic, so what do I know? If the country keeps trading at such unprofitable rates, there's no way we'll be out of this recession in the next five years. But what do I know, I'm only the top economist in the Midwest.

A rhetorical question meant to indicate that one's opinion or claim isn't trustworthy, believable, or welcome. Sometimes used ironically or sarcastically. A: "You really shouldn't be eating so much red meat." B: "Ah, what do you know? Red meat helps make you strong!" A: "All the healthcare experts out there say babies ought to be breastfed exclusively." B: "Bah, what do they know? My siblings and I were all bottle fed, and we turned out just fine." So, what, you think I'm wrong about climate change because of some article you read on Facebook? I mean, I only have a PhD in climatology, what do I know?"