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The American-English noun locker room designates a sports changing-room containing lockers.

This noun is used as a modifier to designate language, attitudes or behaviour associated with, or considered typical, of a (men’s) locker room, especially in being vulgar or coarse.
—Cf. also bathroom humour.

The expression locker-room mentality occurs, for example, in the following from The legacy of George Floyd, about the memories, among Minnesotans, of the murder of George Floyd in 2020, published in The Minnesota Star Tribune (Minneapolis, Minnesota, USA) of Sunday 25^th May 2025 [page C5, column 3]:

LaTroy Hawkins
My brother sent me the video of George Floyd being murdered. I was appalled by the lack of empathy from the other police officers. I understand the “locker-room” mentality, the brotherhood between police officers. But I also know that in a brotherhood, my brother would tell me when I’m wrong. I understand that he’s your superior officer, but where’s your humanity?

These are, in chronological order, the earliest occurrences that I have found of the noun locker room used as a modifier to designate language associated with a locker room:
—Note: This usage originated in golf:

1-: From the Democrat and Chronicle (Rochester, New York, USA) of Sunday 4^th September 1921 [section 3, page 41, column 3]:

Golf Craze Violent in Detroit; Former Rochester Man Tells of Conversation in Locker-Rooms

There are doubtless some wives in Rochester who think that this city is more golf-mad than any other spot in the world. When friend husband stays out at one or other of the city’s golf courses to play another nine holes, while poor wifie stays at home, all dressed up and ready to go somewhere, she probably wishes that the family might move to some other city where there were not so many golf courses within easy reach of the business section of town.
But she should be careful not to suggest moving to Detroit. This city, if one is to judge from The Detroit Golfer, a monthly magazine that is the official publication of the Detroit District Golf Association, is seized more firmly in the toils of the golf craze than Rochester. […]
One of the contributing editors to The Detroit Golfer is William C. Richards, who several years ago was a reporter on the staff of the Democrat and Chronicle. […] In the July issue of the magazine Mr. Richards has two articles: one on the Bloomfield Hills Country Club, in which he sketches accurately and in a delightfully humorous way some of the locker-room talk that may be heard at any country club.

2-: From Hoof and Mouth Corner: Being a Golf Colyum edited by Joseph, Times’ Golf Bug, published in the St. Petersburg Times (St. Petersburg, Florida, USA) of Thursday 6^th July 1922 [page 6, column 6]:

LOCKER-ROOM TALK AFTER GAME AT COUNTRY CLUB

Buck—“I never had such a day. Only time I escaped the rough was on the tee or a green!”
Eddie—“Boy, don’t talk palmetto or rough to me! I only broke my toe trying to cut the grass down. I guess you could load two auto trucks with all the grass I cut!”
Jim—“I’m awful! Simply awful! My new swing don’t work now, it’s about time that little Jimmy takes unto himself to learn game all over again.”
[&c.]

3-: From the column The Razz Berry Bush, by Ted Yudain, published in the Stamford Advocate (Stamford, Connecticut, USA) of Wednesday 31^st October 1928 [page 8, column 3]:

Locker Room Talk

R. V. says that he overheard Chicatelli and Lionne in an “intelligent” conversation the other day . . imagine athletes speaking of Biology . . Lionne: “You remember your biology, huh? . . wut’s a cell?” . . Chic: “Ha, Ha, you kidding me? . . it’s where the [sic] keep prisoners” . . No wonder they have to burn down some of the schools.

4-: From one of the unconnected paragraphs making up a golf column, by Johnny Wolfer, published in The Cincinnati Post (Cincinnati, Ohio, USA) of Saturday 1^st December 1928 [page 5, column 5]:

The latest locker room joke is that a Scotch golfer wrote the song “I Can’t Give You Anything But Love, Baby.”

5-: From The St. Louis Star (St. Louis, Missouri, USA) of Saturday 4^th May 1929 [page 11, column 4]:

Layon McDuffer Is Still Occupied With Mail From New Missaputtans
[…]

Mr. Layon McDuffer, the eminent golfer and president of the Missaputta Golf Club, remained at his desk today for the fourth straight day, even though it was Saturday, generally the day when golf occupied the favored position on his program.
Something more pleasant than golf occupied his time. He was receiving, via one of Uncle Sam’s servants, batches of envelope-enclosed praise; ideas for new rules, statements of eligibility to membership and the inevitable nineteenth hole and locker room joke.

6-: From one of the unconnected paragraphs making up a golf column, by Jerry Diefenderfer, published in The Fort Myers Press (Fort Myers, Florida, USA) of Tuesday 18^th June 1929 [page 6, column 1]:

LOCKER ROOM HUMOR

Dear Editor; In your opinion is Horton Smith a flash in the pan?
Auswer [sic]: Buddy, he’s a whole volcanic eruption.

7-: From the column Chip Shots, by ‘Jigger’, published in the New Britain Herald (New Britain, Connecticut, USA) of Friday 28^th November 1930 [page 21, column 2]:

Miniature golf furnished material for the jokesmiths last summer and continues to provide the funnies with new angles of the ludicrous in human behavior. Putting a rubber ball through dog-houses and over little bridges is rather a ridiculous form of amusement and no one realizes its better than the adults who play midget golf.
But they take it lightly, even if in frequent doses, and very little bragging is heard about the birdies and eagles shot on the miniatures. That feature of little golf stands out in sharp contrast with the locker room talk after the day’s work is finished on a regular course.