To be gullible, naïve, or unsophisticated.
This chiefly rural idiom is often given as fall off the turnip truck but it is most often used in the past tense and preceded by ‘just’. It is often used in the negative as a way of expressing that a person is not foolish, naive, or gullible, as in “I didn’t just fall off the turnip truck, you know.” An equivalent expression is “I wasn’t born yesterday.”
Example
>Mary has no idea about how to act in polite society, always behaving as if she just fell off the turnip truck
> Just because I’m from the country you probably think I just fell off the turnip truck