Christina: Ooh, I like that phone. Can I see it?
Salesman: Sure! And it's a great price! Only $100.
Christina: Wow, that's cheap for a new phone. Is it used?
Salesman: No, it's brand new.
Christina: Then why are there pictures on it of someone's kid?
Salesman: Maybe it fell of the back of a truck.
Christina: I'm sorry - I can't purchase stolen goods. But thanks!
Meaning: When an American says that something "fell off the back of a truck," it means that the item is stolen property. One common usage is like the one above, when a person will state euphemistically that something "fell off the back of a truck" to suggest that something is stolen without admitting to selling stolen goods (which is illegal). In the example above, the salesman is telling Christina that the phone might be stolen, but without admitting anything wrong. However, while the idiom was initially used to euphemistically admit that something was stolen, it is now sometimes used in everyday English to replace "stolen" even though no admission of guilt is being avoided. Here is another example:
Maria: Wow, Daniel! That's a nice bike! Where did you get it?
Daniel: I bought it from a guy in an alley in downtown.
Maria: Did it falloff the back of a truck?
Daniel: I didn't think of that; it was pretty cheap. Think I should go by the police station and see if anyone has reported it stolen?
Maria: Probably a good idea. And who knows - if it is stolen, maybe the owner will give you what you paid for it as a reward.
Meaning: In this case, Maria asks if the bike could be stolen - a possibility Daniel hadn't thought of. Of course, if he gets caught riding a stolen bike, he could get into trouble, so Maria suggests that he check with the police station to find out if it is stolen.