My parents used the phrase “It must have fallen off the truck” all the time. As a child I thought it was something literal. I wondered how or why some of these things actually could be falling from trucks. Why the items didn’t break when they fell, and how anyone could actually get to the item and pick them up once it did fall? The phrase often made me watch trucks to see if I could actually see something falling. I never did.
As I grew older, someone explained it to me. It”s basically a euphemism for any illegially obtained goods or merchandise. I assumed after that, probably correctly, anytime that phrase was used it really meant someone stole it.