Boot is any non like-kind property received by the exchanger (seller) and is taxable to the extent there is capital gain. “Cash boot” is the receipt of exchange proceeds by the exchanger. “Mortgage boot”, also sometimes referred to as “debt relief,” is the exchanger having less debt (loan amount) on the replacement property (property buying) or properties that they had on their relinquished property (property selling). Think of kicking yourself with a boot for paying taxes!