E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Withers of a Horse (The)
are the muscles which unite the neck and shoulders. The skin of this part of a horse is often galled by the pommel of an illfitting saddle, and then the irritation of the saddle makes the horse wince. In 1 Henry IV., ii. 1, one of the carriers gives direction to the ostler to ease the saddle of his horse, Cut. I prythee, Tom, beat Cuts saddle the poor jade is wrung on the withers, that is, the muscles are wrung, and the skin galled by the saddle. And Hamlet says (iii. 2):
Let the galled jade wince, our withers are unwrung.
That is, let those wince who are galled; as for myself, my withers are not wrung, and I am not affected by the bob.