E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
means primarily the hough or pastern of a horse; so called because it bends under, and not over, like the knee-joint. When a horse is lying down and wants to rise on his legs, it is this joint which is brought into action; and when the horse stands on his legs it is these ankle - joints which support him. Metaphorically, voters are the pastern joints of a candidate, whereby he is supported.
A suffragan is a titular bishop who is appointed to assist a prelate; and in relation to an archbishop all bishops are suffragans. The archbishop is the horse, and the bishops are his pasterns.