shank's mare

夏になったので植物の生育が速いので、昨日は出勤前の畑仕事に手間取りブログ投稿の時間が無くなりました。
さて、生物の多様性の保存が大切な事を説いたEdward O. Wilson著のThe Creationを読んでいます。次に引用する個所に出てくる動物 "wolverine" or "Gulo gulo" はWikipediaの写真で見ると熊の一種のように見える稀少動物です。
Its savage demeanor is not, however, why I want to avoid the wolverine. The reason is that I find Gulo gulo the embodiment of wildness, and I know there will still be untrammeled habitats on Earth if wolverines still roam there. I trust they will hold on in the vast subarctic forest, somewhere in North America or Eurasia, in places too far to be reached easily by vehicles or shank's mare.
私が気になったのは "shank's mare" で、"mare" に雌馬の意味があるので馬車の一種かと思いましたが、辞書で確認すると違いました。
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: Fig. travel on foot. You'll find that shank's mare is the quickest way to get across town. Is there a bus, or do I have to use shank's mare?
・Wiktionary: one's own legs, especially as a means of moving from one place to another: The only way we can get there is by shanks' mare.
徒歩のことですか。でも今度は何故これが徒歩の意味になったのか気になります。 後者の辞書には次の様に由来を記述しています。
The expression -- believed to be Scottish in origin -- derives from shanks' nag (shanks-naig 1774), referring to the use of shank to refer to the part of the human leg between the knee and ankle. One theory cites "shank's mare" derived from a horse-drawn lawn mower, manufactured by Shanks & Company Ltd. (founded 1853) which required that the human operator walk behind the device to guide the horse; however, references to the phrase in Scottish literature pre-date the existence of the Shanks lawn mower.
Shanks社製の芝刈り機語源説はもっともらしかったのですが、残念ながらXですね。