Whilst I know what it means, the derivation is not obvious.
Does it have anything to do with the kicking being done by another person now?
Does anybody know of the source of this saying?
Until the 18th century, shoes/boots were made to fit either foot, so if the boot/shoe was uncomfortable on one foot, it could be tried on the other. Once shoemakers created lasts which allowed the footwear to fit left and right feet, wearing a boot/shoe on the wrong foot became uncomfortable.
The boot is on the other foot 
Linear Mode
