(Oxford English Dictionary)
Also attrib., esp. as drug-store cowboy, a braggart, loafer, or good-for-nothing; a person who is not a cowboy but is dressed like one.
1925 College Humor Feb. 57/1 (heading) With the Drug Store Cowboys.
(Historical Dictionary of American Slang)
drugstore cowboy n.
1. an idle young man who lounges at or near a drugstoe doa fountain, esp. for the purpose of socializing with young women; (broadly) a soft or callow fellow.
1923 T.A. Dorgan, in Zwilling TAD LEXICON 33 [July 26]: Hangin' around that gang of drugstore cowboys.
31 October 1922, Mexia (TX) Evening News, pg. 3, col. 3:
Drug Store Cowboy
Latest Name For
Young Jelly Beans
OKLAHOMA, Okla. Oct. 31 -- The "Drug Store Cowboy" is the latest classification given local "faddish" young men.
Besides "Jelly-bean" and "Cake-eater," "Drug Store Cowboy" will go down in the book the ever changing younger generation.
The dress of the "Drug Store Cowboy" is distinctive as is that of the "Jelly-bean" and "Cake-eater."
The biggest and fussiest cow-puncher John B. available, classy boots, and in some instances the real thing -- high heeled rough leather boots, a classy wool shirt of the right shade of blue or gray, and a neck scarf with lots of cowboy lingo compose the apparel distinctive of the "Drug Store Cowboy."
1 September 1923, Evening State Journal (Lincoln, NE), pg. 6:
If there was any justice in this world outside of Sears & Roebuck's brand of auto tires the drug store cowboys (all of them) would be out taking up space in the institution just this side of Yankee Hill.
17 November 1923, Atlanta (GA) Constitution, "Bits of New York Life" by O. O. McIntyre, pg. 6:
Cheap sports with red-rimmed eyes, drug store cowboys, girls with immobile masks of red and white, old white-moustached dogs, cookies and gambling house runners. It is a swirling eddy of Broadway's scum -- the promenade for those who live by their wits.