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.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Suffrage from Fact Monster: