A euphemism for ‘acquired illegally’.
When anyone accounts for their possession of an article by saying it ‘fell off the back of a truck’ or ‘fell off the back of a lorry’, they may be assumed not to be its legal owner – i.e. it is stolen. ‘Lorry’ is the British version; in the USA and Australia things fall from trucks. This coy language, which feigns innocence but actually emphasizes illegality by using a phrase that is reserved for illegal dealing, is similar to The Godfather’s ‘an offer he can’t refuse‘.