Be born yesterday

I was not born yesterday, you know. I have done this before!

"Do you think," he added, with an ironical grin, "that you'll go for to gammon me into that air! I'm hinnocent, I know, but I wasn't born yesterday exactly."

"Say, do I look like I was born yesterday? See any green in my eye, Cactus Center?"

"Compared to me, you were born yesterday."

If you don't know important things about history, then it's as if you were born yesterday.