With allusion to the phrase the oldest profession in the world, which designates prostitution, the phrase the second-oldest profession (in the world) is used of a profession, trade, art, etc., which has long been established or which is regarded as having similarities with prostitution.

However, the second-oldest profession (in the world) is also sometimes used jocularly, no matter how long what this phrase designates has been established or whether it has anything in common with prostitution.

The phrase the second-oldest profession (in the world) occurred, for example, in Rip-off publicans?, by Linda Daly, published in the Irish Independent (Dublin, County Dublin, Ireland) of Thursday 10^th May 2007 [Vol. 116, No. 111, Supplement You need your local (Vintners’ Federation of Ireland), page 5, column 3]—Linda Daly quoted Tadg O’Sullivan, CEO of the Vintners’ Federation of Ireland:

“The one thing I will say about the pub trade is that it has survived flood and famine, war and changing governments and it will survive the present difficulties as well. The pub trade is the second oldest profession in the world. It has survived all of these troubles throughout the years. Yes, there have been peaks and troughs but, at the end of the day, the pub and particularly the Irish pub, has something unique to offer and as a result it will survive.”

The earliest occurrences that I have found of the phrase the second-oldest profession (in the world) are as follows, in chronological order:

1-: From The Brooklyn Daily Eagle (Brooklyn, New York, USA) of Friday 5^th November 1915 [Vol. 75, No. 307, page 15, column 3]:

Inspiring accounts of the deeds and days of the buccaneers, pirates, filibusters, highwaymen, privateers and freebooters—“amiable gentlemen” all of them—were told to a group of New York City’s distinguished and law-abiding citizens last night in the Grolier Club rooms, at 29 East Thirty-second street, Manhattan. And supplementing the stories of those bold corsairs which were told by Don C. Seitz * of Brooklyn, all the known literature ever written on the subject, including many rare and original maps, parchments and documents, were put on exhibition at the clubrooms.
[…]
Mr. Seitz, telling of the stories of those adventures, “lawful and unlawful,” stated that piracy is the second oldest profession in the world.
“True, it has been stamped out,” said Mr. Seitz, “but the highwaymen we still have with us. The newspapers have succeeded the old ballads and broadsides on highwaymen.”

* Don Carlos Seitz (1862-1935) later wrote Under the Black Flag—cf., below, quotation 6.

2-: From The Boss of Mirage: A Complete Novelette, by E. S. Pladwell, published in Adventure (New York: The Ridgway Company) of Sunday 3^rd August 1919 [Vol. 22, No. 3, page 9, column 1]:

Black had a reputation.
THE man belonged to the second oldest profession in the world. Back in the shadowy ages of antiquity the kings of a legendary Egypt employed his predecessors to do their bidding. Cambyses, Claudius and Nero used them lavishly. The Byzantine emperors posted them on every street corner, and the age of the Borgias lifted their craftsmanship almost into the arts. From there they slowly declined in numbers and power until today they are found only in the toughest wards of the big cities or on the last frontiers.
“Silent Jim” Black was a professional killer.

3-: From The Stranger Within Our Gates, by ‘Roped In’, published in the Derby Daily Telegraph (Derby, Derbyshire, England) of Wednesday 21^st July 1920 [Vol. 70, No. 12,735, page 2, column 5]—‘Roped In’ has met an odd old man who was prowling about the Market Hall, and who tells him:

“The world should be kept moving, and it is a mistake to suppose that money makes the world go round—a few quarts of beer make it spin like a top. I am in sympathy with the Labour movement—anything that keeps labour moving from me has my deep sympathy. I am a milestone inspector by profession—”
“Excuse me,” I interrupted, “but what the — that.”
“That,” he replied rather surprised, “is the second oldest profession in the world—it is called a daily inspection of landmarks.”
“In other words,” said I, “you just loaf around.”
“That in itself is an art,” said he. “You can get more tired and thirsty doing nothing than —”
“I’ve noticed your thirst,” said I.

4-: From The Fourth Column, published in The Border Cities Star (Windsor, Ontario, Canada) of Saturday 19^th May 1923 [Vol. 10, No. 68, page 4, column 4]:

BANDITS IN CHINA

So far as China is concerned, banditry is probably the second-oldest profession, and those who are astonished at its recent manifestation in Shantung are not aware of Chinese conditions.

5-: From the column New York Day by Day, by O. O. McIntyre (1884-1938), published in several newspapers on Wednesday 23^rd January 1924—for example in The Sioux City Journal (Sioux City, Iowa, USA) [Vol. 54, No. 216, page 6, column 4]:

New York, Jan. 22.—It has been several years since Richard Canfield, dubbed at police headquarters as a “common gambler,” met his death by a fall in the subway. Yet each year a group of men get together to revere his memory. They are not gamblers or sports, but men who have a reverence for art.
Canfield, despite his calling, which he said was the second oldest profession, is considered by many as one of America’s greatest critics of art. Whistler thought so much of him he painted his portrait, which, with his tongue in his cheek, he called “His Reverence.”

6-: From Under the Black Flag (New York: Lincoln MacVeagh, The Dial Press, 1925), by Don Carlos Seitz (1862-1935) [page 3]:

PIRACY is the second oldest profession in the world.

7-: From A Bullet of Mercy, an unsigned short story published in the Daily News (New York City, New York, USA) of Monday 6^th July 1925 [Vol. 7, No. 9, page 19, column 3]—in this short story, set in New York City, a good-looking young man, called Foxy Harris, approaches Masie Parker, “a sweet, inexperienced girl who [has] recently come to New York”:

They strolled down the Boardwalk together—a man and a girl.
Now all was grist that came to Foxy’s mill, but this looked like a peculiarly easy handful of grain. He knew his stuff, did Foxy—and he hadn’t been with Masie ten minutes before he knew all about her. What he heard made his heart glad. She was ideal for his purpose.
“Fool dame from a tank town—no folks—nobody to worry much about her,” thought Foxy. “Pretty fair lookin’ if she was dressed up right. This is goin’ to be easy. Mother Walsh will be tickled pink when I bring this chicken in.”
For Foxy, be it known, belonged to the second oldest profession in the world. And, while it must be admitted that the senior profession is shameful, that of Foxy Harris was many times more reprehensible. Foxy was one of that carrion flock which lives off the earnings of those unfortunate little sisters of mischance.

8-: From the column Manhattan Days & Nights, by Herbert M. Corey, published in the Pasadena Evening Post (Pasadena, California, USA) of Thursday 8^th July 1926 [Vol. 7, No. 274, page 12, column 3]:

I am told that the second oldest profession in the world—that of “fencing” stolen goods—has been oddly changed through the researches into human nature conducted by the bootleggers at some cost to the rest of us. Time was when a fence wore a greasy frock coat, probably had only one eye, and was approached at midnight through a hole in the back fence. Now—
“Like to buy a dressing gown—or golf clubs—or imported shirts?”
He names an absurd price and eventually the goods are delivered as per contract. Explanation is that no theft is made until the sale has been arranged. Saves warehousing and trouble.

9-: From One, Two, Three (New York: George H. Doran Company, [1927]), by the British author and translator Paul Selver (1888-1970) [page 200]—the following takes place during an interval at the première of a stage play entitled One, Two, Three:

He now found himself being introduced to a number of people, towards whom he at once conceived a violent enmity. […] There was much noisy and high-pitched gab from two young gentlemen, whose appearance suggested that they belonged to the second oldest profession in the world (or was it the third?). Their conversational efforts were run very close by a young lady who thought, and announced with great vocal emphasis, that the first act of One, Two, Three was too thrilling for words, but she found words, and many of them, nevertheless.

10-: From the transcript of a debate on the Architects (Registration) Bill that took place in the House of Commons, published in The Evening News and Southern Daily Mail (Portsmouth, Hampshire, England) of Friday 8^th April 1927 [No. 17,337, page 14, column 1]—Here, the oldest profession designates agriculture:

No “Art by Registration.”
Major Hills, opposing the measure, said they did not get the best art by registration. Wren was never an architect, and if the Institute of British Architects had existed in his time he could never have practised as an architect under the Bill. No art would flourish in chains.
The Second Oldest Profession.
Mr. Gardner, supporting the Bill, denied that the Institute of British Architects was a close corporation, as had been suggested by the previous speaker. The Bill would protect the public from rapacious contracts and shoddy work, and give the oldest profession, except that of agriculture, the recognition it deserved.

11-: From The Bar Sinister, published in The Idaho Daily Statesman (Boise, Idaho, USA) of Friday 22^nd July 1927 [No. 311, page 4, column 1]:

Our supreme court has added to the privileges of this preferred class by deciding that a lawyer may send obscene and unmailable matter through the United States mails and still be a gentleman and a lawyer.
The supreme court cannot be wrong. Possibly we can now grant to the legal profession the position of being the second oldest profession in the world.

12-: From Plenty of Rotten Booze Is Inspiration for Thugs In Pennsylvania Coal Fields, by Art Shields, published in the Oklahoma Leader (Oklahoma City, Oklahoma, USA) of Friday 18^th November 1927 [Vol. 9, No. 14, page 5, column 1]:

PITTSBURGH, Pa.—Some say the marines won the war, and other doughboys insist it was French cognac. But if you smell the breath of the coal and iron police in the Pittsburgh mining region you’ll say its [sic] pretty rotten moonshine they are relying on. Pretty rotten, but lots of it. Business may be bad in other lines except bootlegging, but the thirsty gullets of the “Yellow Dogs” are keeping the practitioners of the second oldest profession out of the poorhouse.