Why do people say "I'm no [insert profession here], but [make claims like an expert]?"

I'm no historian, but I can tell you that this place was important.<br><br>I'm no doctor, but I'm sure you have a stomach bug. <br><br>I'm no chef, but I'm going to tell you the RIGHT way to do this. <br><br>I'm no mechanic, but holy shit, your parking sucks.