just fell off the turnip truck

If someone says they just fell off the turnip truck it means that they are ignorant or unsophisticated as these are the things a turnip truck is most related to. However most of the time this phrase is used in the negative where as they would say they did not just fall off the turnip truck which means they want you to know they are not unsophisticated, ignorant or not intelligent enough to understand the current situation.