The expression -- believed to be Scottish in origin -- derives from shanks' nag
1774), referring to the use of shank
to refer to the part of the human leg between the knee
. 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.