The other day someone said to me "I didn't just fall off the turnip truck, ya know", which made me chuckle. When I was a kid my mother always said, "I didn't come up the Clyde on a water biscuit you know",(I am a Glaswegian of course) and I often say, "I'm not as green as I'm cabbage looking" which is another one of those 'I ain't no rube' type sayings.
Just wondered what similar phrases you use in you neck of the woods?
Giok