What is the origin of the idiom "off the back of a truck" and when to use it?

The idiom "off the back of a truck" originated from the idea of stolen or illegally obtained goods being sold or acquired from the back of a truck. It is often used in informal conversations, particularly when discussing items that appear suspicious or are obtained without proper documentation or authorization. The phrase is used to convey a sense of skepticism or doubt about the legitimacy of the goods in question.