In The Shakespeare Stealer, the term “shank’s mare” is used only once, on p. 20.  At that point, the meaning of the phrase is given clearly.  Widge, the narrator, says that “shank’s mare” meant “of course, my feet.”  The phrase “shank’s mare” was used in English from a relatively long time ago to mean one’s own legs.  To travel by shank’s mare was to walk.

In Shakespeare’s time, the word “shank” was used to refer to a person’s lower legs.  The part of the leg between your knee and your ankle was called your shank.  A mare, of course, is an adult female horse.  The phrase “shank’s mare,” then, was supposed to be a humorous one.  It was saying that your legs were your horse, which means, of course, that you had no horse at all and had to walk on your own.

While The Shakespeare Stealer is set in Shakespeare’s time, Shakespeare himself never used the phrase “shank’s mare” in any of his writings.  It apparently was not used enough to show up in writing until much later.  However it sounds archaic and very English to modern Americans and therefore gets used in books set long ago in England.