Of American-English origin, the adjective pin-up first appeared in pin-up girl, denoting a woman being the subject of a picture that a serviceman displays on a locker-door, on a wall, etc. The earliest instance that I have found is from the caption to this photograph, in the East Liverpool Review (East Liverpool, Ohio) of 5^th July 1941:

PEEKABOO. The four principals of “Caught in the Draft” seem to be hiding from something—maybe the colonel. The Columbia theater has this slightly wacky Bob Hope comedy as the week-end attraction. Dorothy Lamour, the Army’s No. 1 “pin-up” girl, Lynne Overman, Eddie Bracken and Clarence Kolb head the cast.

In his column for the St. Louis Star-Times (St. Louis, Missouri) of 11^th September 1941, Bill Cunningham explained:

Here’s a rough outline of the script of the greatest radio show ever hurled into the ether for any cause whatsoever.
[…]
The next-to-final smash will be girls. We’ve gone into the sarong and sweater sorority of Hollywood and recruited the four goddesses of glamour, supposed to be the favorite pin-up girls of this man’s war, meaning the girls whose pictures are pasted in more lockers than any others. These are Ann Sheridan, Rita Hayworth, Lana Turner and Dorothy Lamour.

The following is from the Miami Daily News (Miami, Florida) of 14^th November 1941:

Pin Up Girl
Open warfare today broke out between two sections of Uncle Sam’s armed forces.
Dorothy Lamour may be the army’s “No. 1 ‘Pin Up’ girl,” as was recently announced by “Life” magazine, but the navy isn’t going to give her up without a struggle.
The Paramount star, currently at work in a navy picture called “The Fleet’s In,” has been so informed by the crew of the U. S. S Dixie, which requested enough pictures so that every member of the Dixie’s crew could “pin up” their favorite star in their lockers. And the midshipmen at Annapolis recently received 500 photos.

 

ILLUSTRATIONS

 

from the St. Petersburg Times (St. Petersburg, Florida) – 14^th February 1943:

This Week’s Pin-Up Girl
(To Decorate the Soldier’s Room or Tent)

Maureen O’Hara supplies the love interest in “The Immortal Sergeant,” based on the Libyan campaign in North Africa. It is an authentic picturization of desert warfare in World war II.

 

from The Pittsburgh Courier (Pittsburgh, Pennsylvania) – 5^th June 1943:

THIS WEEK’S “PIN-UP GIRL” IS A CHARMER DᴱLUXE

Beautiful and talented Anna Mae Winburn, the Theatrical Department’s selection for this week’s “Something for the Boys.” Leader of the nationally famous “Sweethearts of Rhythm,” Anna is herself a typical sweetheart . . . not hard on the eyes for any guy. Anna has been selected by the boys in the 24^th Infantry band, “Somewhere in the Pacific,” as honorary conductor of their fine crew, which is led by Staff Sgt. Harvey Rhodes, in the absence of Warrant Officer Robert Tresville Sr., who has returned to the States.

 

from the Muncie Evening Press (Muncie, Indiana) – 20^th September 1943:

Scotch Pin-Up Girl Adopted By Yank Flyers In England

Moyra, one of the hundreds of British war orphans “adopted” by American soldiers, inspects her namesake before christening the plane.

 

from the Minneapolis Sunday Tribune (Minneapolis, Minnesota) – 12^th December 1943:

SPEAKING OF PIN-UP GIRLS . . . .

Here are the faces of 12 women whose pictures might well be enshrined in barracks of the armed forces everywhere. They are representative of thousands of women war workers who keep supplies rolling to the war fronts throughout the world.

Mrs. Dorothy E. Nelson, 4240 Third avenue S., a bullet machine operation inspector, is the pin-up girl of her husband, Cpl. Douglas Nelson, who has been in Aleutians for 18 months.