This term is usually used in Australian and American English, and the British are used to saying fell off the back of a lorry more often, though sometimes they employ the American phrase.

When the term appeared in the USA in the early 20th century, it always referred to something shoddy or trashy. Later the meaning changed, and nowadays the phrase is used to describe any illegally obtained item. You can also sometimes hear the phrase literally fell off the back of a truck in news stories, where it signifies that the report is not referring to a stolen item or stolen items.

An example:

Sometimes people consciously buy things that fell off the back of a truck.

And with the second phrase:

"It Fell off the back of the Truck"....Literally! Bread for everyone!"

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