It may refer to the return of Sunday, which happens in TMWWT, or
It may refer to the anatomical part of Sunday which is opposite his front.
In TMWWT much is made of both possibilities. Sunday is feared throughout most of the story, and when he his seen, it is his back which generates the most comment.
Several of the participants in this book discussion, and hopefully many more who are reading the posts and the book as well, have discovered that TMWWT is not what it seemed after being read the first time. Often the reaction at that point is aggravated befuddlement. But by then the story is in one's mind and the slightly uncomfortable feeling that something was missed begins to grow. If one gives into that feeling a second trip through the story often is the result during which many unnoticed things reveal themselves and the plot begins to unfold. It was only on my fourth exposure that I connected the lilacs and Rosamond from Chapter 1 with the concluding paragraph of the story where Rosamond is observed cutting lilacs; and the use of the word grave regarding her and her face. I needed Suzanne's help to see some of this so I encourage you to post on this discussion especially questions. There is much more to TMWWT than can be garnered without at least some reflection and no one has all the answers. But that's what makes a book fun isn't it? Discovering that there is more to unravel than, 'the butler did it."
Chesterton trained to be an artist and he initially gained notoriety by reviewing art for publications. His artistic training allowed him to paint some vivid word pictures in his books and they are certainly present in TMWWT.
Chesterton also brought his artistic training to bear on his character descriptions. In TMWWT he expends a great deal of effort describing the Back of Sunday. Here is what Chesterton said in a review of G.F. Watts painting Eve Repents ca 1875.
Before we quit this second department of the temperament of Watts, as expressed in his line, mention must be made of what is beyond all question the most interesting and most supremely personal of all the elements in the painter's designs and draughtsmanship. That is, of course, his magnificent discovery of the artistic effect of the human back. The back is the most awful and mysterious thing in the universe: it is impossible to speak about it. It is the part of Inan that he knows nothing of; like an outlying province forgotten by an emperor. It is a common saying that anything may happen behind our backs: transcendentally considered the thing has an eerie truth about it. Eden may be behind our backs, or Fairyland. But this mystery of the human back: has again its other side in the strange impression produced on those behind: to walk behind anyone along a lane is a thing that, properly speaking, touches the oldest nerve of awe. Watts has realized this as no one in art or letters has realized it in the whole history of the world: it has made him great. There is one possible exception to his monopoly of this magnificent craze. Two thousand years before, in the dark: scriptures of a nomad people, it had been said that their prophet saw the immense Creator of all things, but only saw Him from behind. I do not know whether even Watts would dare to paint that. But it reads like one of his pictures, like the most terrific of all his pictures, which he has kept veiled.
http://gutenberg.net.au/ebooks12/1201851h.html
