Where Is It

Alright, listen up, you're clearly not the sharpest tool in the shed. So, the guy thinks he's all that showing off his junk to his girlfriend, right? But she's basically saying, "Dude, that thing is so small, I can't even see it." Get it? It's an insult hidden as a question. It's funny because of how unexpected it is, and because it's about a guy's small "thing" which is typically a sensitive subject.