To understand why this phrase is incorrect, you first have to understand what it means to "pawn" something. A pawn shop owner is basically a low-level lender: One pawns an item -- electronics, musical instruments, microwaves -- to the owner (also known as a pawnbroker) in return for money. The person who pawned the item can get it back if he pays back the money, with a certain amount of interest, within an agreed-upon time limit [source: National Pawnbrokers Association].
Thus, the phrase "pawn off" should be used only when referring to trading an item as collateral for a loan. That hasn't stopped people from confusing the saying with the similar-sounding "palm off," which means to get rid of something or someone by means of deception [source: Macmillan Dictionary]. This phrase likely came from card-playing and concealing cards in the palm of your hand.