Some restaurants have a dress code and require men to wear jackets. Instead of turning an inappropriately dressed customer away, the restaurants often have “loaner jackets” that a customer can borrow while dining in the restaurant.
 
“Loaner jacket” has been cited in print since at least 1987. “Dressing Up the Loaner Jacket” was a story in the New York (NY) Times on August 19, 2013. Although loaner jackets usually had been low quality, the Times reported that a few restaurants carried a large selection of high quality jackets to meet the demand from inappropriately dressed customers.
 
 
15 May 1987, Washington (DC) Post, “Dining Out on `Kangaroo’; A Reluctant Repast With Judy Davis & Colin Fields, the New Film’s Stars.” by Mary Battiata, Style, pg. D1:
Friels, sitting across from her in jeans and loaner jacket, looked like a rough- hewn country cousin.
 
21 March 1988, Washington (DC) Post, “Finery for the Dinery; Tux or T-Shirt, the Code Truth About Restaurants” by Martha Sherrill Dailey, Style, pg. C5:
At the John Hay Room, the fanciest restaurant in the Hay Adams Hotel, coats and ties are required, but according to Chip Brennan, director of food and beverage, they rarely have to turn anyone away or hand out loaner jackets.
 
3 October 1998, Salina (KS) Journal, “Dressing Up for Dining Out: Is it dying?” by Lisa Jennings (Scripps Howard News Service), pg. A5, col. 4:
Loaner jackets available
At Erling Jensen: The Restaurant, another special occasion spot, jackets are requested but not required. If a customer feels uncomfortable about being inappropriately dressed, a loaner jacket will be offered, said co-owner Kathy Dempsey.
 
Google Books
The Biggest City in America
A Fifties Boyhood in Ohio
By Richard B. Schwartz
Akron, OH: University of Akron Press
1999
Pg. 83:
It required jackets of the gentlemen who patronized it, but the loaner jackets which it offered (“Perhaps you could try on one of ours . . .”) were straight out of the Moose Lodge, with light pastels warring on the rack with broad plaids.
 
Google Books
I Love Huckabees:
The Shooting Script
By David O. Russell abd Jeff Baena
New York, NY: Newmarket Press
2004
Pg. 4:
FANCY RESTAURANT - DAY
The maitre d’ helps Albert put on the loaner jacket.
   
Washington (DC) Post
February 12, 2009; 8:36 AM ET
Raw Fisher:
Is D.C. Still A Jacket & Tie Town?
By Marc Fisher
(...)
To get to “yes,” Georgetown’s 1789 Restaurant keeps loaner jackets for those who arrive in shirt sleeves. “Every six months, we have to go out and buy more blazers,” says General Manager William Watts, because some gents forget they’ve borrowed the coat and wear it home.
 
New York (NY) Times
Dressing Up the Loaner Jacket
By HELENE STAPINSKI
Published: August 19, 2013
It was a humid day, but every man seated in the Grill Room of the Four Seasons was wearing a jacket.
(...)
David Gooch, a manager of Galatoire’s in New Orleans, used to keep about 10 jackets on hand. Ten years ago, the number climbed to 25. Now he uses as many as 40. Sometimes in summer, when more people tend to walk around without a jacket, Mr. Gooch runs out for more.
 
And not only are there more jackets, but the ones on hand are also far nicer than before. They come in an array of sizes, so the diner won’t look as if he is wearing his father’s suit jacket, or his little brother’s. At the Four Seasons, there are even two children’s blazers available for shirt-sleeved juniors.
     
The Times-Picayune (New Orleans, LA)
Loaner jackets are on the menu for the underdressed at Galatoire’s
By Doug MacCash, NOLA.com
on August 23, 2013 at 1:05 PM, updated August 24, 2013 at 9:20 AM
Gentlemen, if you’re going to dine at Galatoire’s restaurant after 5, you’re going to have to wear a dinner jacket. Period. No exceptions. As a recent New York Times story pointed out, the classy Creole landmark is one of the few dining establishments across the land that still strictly enforces a dress code.
 
Not that you’ll be turned away if you arrive without your own jacket. There’s a selection of 20 to 30 somberly colored jackets in a two-tiered rack near the door.