Meaning of the idiom "fall off a turnip truck"

[Zishuli]

English Language Tests, Intermediate level

ESL/EFL Test #346 "English Slang Idioms (67)", question 2

"You don't buy that lame story, do you? She is obviously lying to you. How can you not see that? Did you just fall off a turnip ........., or something?" His best friend asked.

(a) pile
(b) garden
(c) shelf
(d) truck

English Language Tests, Intermediate level

ESL/EFL Test #346 "English Slang Idioms (67)", answer 2

"You don't buy that lame story, do you? She is obviously lying to you. How can you not see that? Did you just fall off a turnip truck, or something?" His best friend asked.

Correct answer: (d) truck

Your answer was: correct
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Hi,

I am not quite sure that what the meaning of " fall off a turnip truck" is.

Help me, please.

[Mister Micawber]

.
It is an idiom.

Did you just fall off a turnip truck? = Are you naive/unsophisticated?
.
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Native English teacher at Mister Micawber's