A form of
public transport that is available to most people completely
free of charge. The unfortunate
downsides are that it's slow and with
standing room only.
It is, in short, a euphemism for walking, born in Middle English times when the horse was the only better alternative for getting from A to B.
The word 'shanks' has a number of different definitions including the area of the legs between the knees and ankles or, for the shoemaking trade, the part of the sole beneath the instep connecting the broader front part with the heel.
Apparently also known as Shanks' Mare.