about " turnip truck"

[english-learner]

What does " turnip truck" mean in the context? Thanks!

MANDY
[on the phone] Bruce? Bruce? Bruce! I may have just gotten back into the business this morning, but I didn’t come by way of a turnip truck, you know what I’m saying? [pause] You jerk me around on this, and I’m going to get cranky right in your face. Now, I was your source on 443. Big, fat byline. Above the fold. So, I think it’s time to play ‘What have you done for me lately.’ Look, I don’t want to hear you’re going to try, Bruce. This isn’t gym class.

[Robert B. Mercer]

I did not come by way (I did not arrive riding on top of) a turnip truck (a farm truck carrying lots of the vegetables, turnips),

or, what I always hear:

I didn't just fall off of a turnip truck...

It means:
I am not stupid... I am not stupid like a child... you think I know nothing, but I am smart... you are trying to pretend you are smarter than me, but you are not....

Why?
People in the city (sometimes) think people who live on farms are stupid. So, the sentence is saying:
"I did not come from a farm, riding here on top of a truck carrying vegetables like a stupid person."


Other phrases like this:

I wasn't born yesterday!
I'm not as dumb as I look!
I'm not as stupid as I look!
I didn't just get off the boat! (I did not just arrive here)

Or, you can ask the question:

Did you just fall off the turnip truck? (Are you as stupid as a farmer?)
Did you just get here by boat? (Are you new here and stupid?)
Are you as dumb (stupid) as you look?

[english-learner]

Thanks a lot again. I am really impressive about your explanation to my question. It is so wonderful. I wish you would help me my questions every day.

So I can put this sentence this way, is that right?

I didn’t come by way of a turnip truck, you know what I’m saying?= I am not stupid and you know what I am talking about.