... I loved shoe salesmen. Not the dispirited store clerks they have now, but actual shoe salesmen who knew their business.
Yes, I'm talking about real shoe men who knew shoes and feet that went in them with an expertise that rivalled that of NASA engineers.
The Road Gets Longer If I Stop (Lulu.com, 2013)
1961 | Vero Cuoio ad detail (Genuine Leather)
Source: Calzature Italiane di Lusso magazine
That, in a nutshell, is the definition of a "shoe dog", also "shoe-dog" or "shoedog", any way you wanted it.
The key things are the dedication and the expertise of the craft. The obsession, also.
To further illustrate the term, take novelist George Pelecanos: back in 1994 he placed a shoedog in the novel of the same name:
“That skinny kid, at the store—”
“Yeah. He called you ‘Shoedog’. You gonna tell me now what that’s all about?”
“You might not understand, man. It’s about having some kind of direction in your life.”
Randolph leaned over the table. “You ever see a dog, man, when he’s walkin’ across a bridge? Well, that dog, he doesn’t look left and he doesn’t look right. He keeps his head down, lookin’ at his paws makin’ a straight line, all the way. And the only thing he’s thinking about, the whole time, is gettin’ to the other side of that bridge.”
George P. Pelecanos
“Shoedog”. (Hachette, 1994)
Hope this is clear. If not - or even if it is - the novel "Shoedog" comes highly recommended.
Hideous lettering and layout | Don't judge this book by the cover
George P. Pelecanos | Shoedog (Hachette, 1994)
More recently the term "shoedog" gained notoriety thanks to Nike's co-founder Phil Knight's memoir:
“Shoe dogs were people who devoted themselves wholly to the making, selling, buying, or designing of shoes. Lifers used the phrase cheerfully to describe other lifers, men and women who had toiled so long and hard in the shoe trade, they thought and talked about nothing else. It was an all-consuming mania, a recognizable psychological disorder, to care so much about insoles and outsoles, linings and welts, rivets and vamps.”
From: “Shoe Dog” (Scribner 2016)
Shoe Dog (Scribner 2016)
We cannot say when the term came to life; we do know however that the oldest use of the word in novels (that we know of) comes from beloved/hated critic Leslie A. Fiedler in his short stories collection "Pull Down Vanity, And Other Stories".
“I am a shoe salesman, a shoe-dog my colleagues prefer to say, too frail and timorous to work ever in the great downtown stores, where each pants leg is creased to an ultimate sharpness, each skull, ferociously brushed and brilliantined, shines in the subdued light, and the quick crackle of conviction spreads from the insolent smile, the jaunty stride, noiseless on the thick carpeting."
Leslie A. Fiedler
From the short story "An Expense Of Spirit" off “Pull Down Vanity, And Other Stories”(J.B. Lippincott Company, 1962)
The short story was first published by the magazine Partisan Review in January 1949 (available online at Howard Gotlieb Archival Research Center); Fiedler also used "shoe dog" again in another story called "Nobody Ever Died from It" (1956), also collected in "Pull Down Vanity".
The author doesn't delves much into the word meaning, but the book highlights a few passages that shine a light into it. Maybe, he too, was a shoedog after all. 
1921 | I.Miller | Deluxe Shoes
In November 2010, The New York Times run an article titled "American Speech: Lingo of the Shoe Salesman (1934)". It was taken from the archive of American Speech – “a quarterly of linguistic usage” published on behalf of the American Dialect Society.
It goes to a great length to explain terms such as "up", "McKay", "wrap-up", "chromo","compo", and so on; unfortunately when it comes to our term - at the very end of the article - we got a laconic explanation:
From what has been said in the foregoing paragraphs, it may have been noticed that the shoe salesman’s lot is not a happy one. This perhaps accounts for his name. In the lingo he is known as a shoe-dog.
American Speech - Vol. 9, No. 4 (Dec., 1934), pp. 283-286
Yes, perhaps. Or perhaps not. Shoe salesmen may not be very happy, but that it might apply to NASA engineers, runway models or X Factor judges. Especially X Factor judges, so it doesn't add up. Good thing is the date - 1934 -, so that we know "shoe dog" was already there.
1957 | Sidney's David Jones' store
Feat. I.Miller shoes
Leslie A. Fiedler was indeed a shoedog:
“As mesmerized as he may have been by imaginative literature, Leslie got his earliest knowledge of the world through real-life experience. Like so many of his fellow Jewish intellectuals, who would later constitute a major force in twentieth-century American criticism, Leslie earned his first paychecks doing nonintellectual work.
His Saturdays at the shoe store his uncle managed would begin at eight in the morning and last until he and his fellow workers had cleaned the place at eleven or eleven thirty at night.”
Mark Royden Winchell
From: “Too Good to Be True: The Life and Work Of Leslie Fiedler” (University Of Missouri Press, 2002)
Time to "wrap-up", but if you happen to know more about the word "shoedog", please let us know.
ca. 1911 | Elizabeth Sparhawk-Jones
The Shoe Shop
Source: Art Institute Chicago