“I didn’t just fall off a turnip truck.” Is a phrase used in Mississippi. I was told that its meaning is somewhat akin to “I wasn’t born yesterday.” An example the informant gave of its use was that when lying to one’s parents about something, they may respond that they didn’t just fall off a turnip truck in order to indicate that you aren’t fooling them. The informant mentioned that it may have come about because turnips were commonly eaten in the area, though felt that a bit strange, as the informant theirself didn’t recall eating turnips regularly.
My best guess of where this proverb came from is that turnips being transported on trucks were a common sight in that region. Turnips are smaller, more compact vegetables, and so more likely to fall from a truck than some other vegetables, and according to my informant may be more commonly seen in the area than others. The turnips that fell from trucks were likely not going to be retrieved by the truck driver, and so were now subject to the joking law of “Finders Keepers” This gives the connotation that a turnip can be easily used/taken, which leads you to the meaning in the proverb being a distancing of oneself from this easy manipulatability. It also helps that the simple action of falling from something can leave someone dazed/dizzy which would leave them easy to confuse, which is another thing the proverb distances the speaker from.