fall off a truck

Of goods or merchandise, to be acquired by illegal or dubious means; to come into (someone's) possession without being paid for. Danny says he has several laptops and smartphones he wants to sell for cheap—sounds like they fell off a truck to me. A: "Jake's been peddling a bunch of flat screens for a great price." A: "They probably fell off a truck then. I wouldn't go for them, if I were you." These prices seem a little too good to be true. You sure this stuff didn't fall off a truck?