The storage compartment at the back of a car, what Americans call the trunk. You’ll hear it in everyday chat, especially when someone’s doing a shop or hauling gear, like pop it in the boot or chuck it in the boot. Sounds like footwear, sure, but in context nobody’s thinking about your shoes.
"Chuck the esky in the boot, we’re off down the coast. After that Bunnings stop, it’s already packed to the gills, mate."