“Does Macy’s tell Gimbels?”

Macy's is the World's Largest Store, at 34th Street and Herald Square. in New York City. Across the street on 34th Street was once a rival store called Gimbels.

"Does Macy's tell Gimbels?" means "does one rival business share its secrets with another?" The saying has been cited in print since at least June 1930.


Wiktionary: does Macy's tell Gimbel's
Etymology
Macy's and Gimbel's were popular department stores located within two blocks of one another in Midtown Manhattan in New York City; the latter store is now defunct.
Proverb
Does Macy's tell Gimbel's?

1. (US, dated, colloquial, rhetorical question) A rhetorical question with the implied answer being that competitors do not share business secrets with one another.
Usage notes
Popular from the 1930s into the 1960s. Now used chiefly among older people.

12 June 1930, Akron (OH) Beacon Journal, "On Broadway" by Walter Winchell, pg. 21, col. 1:
NOT NEW BUT CLEAN!
Joe Hanratty of the H. Liveright offices would have you believe he actually overheard it in the Go-Ghetto district, where there is a pushcart peddler for every broken heart.

Two pushcart peddlers were pushing their pushcarts along the congested area when the one on the left, whose name was Moe, said to the other, whose Christian name was Ben: "Nu, howz business?"

Ben made no reply.

"I sad," reiterated Moe, "Howz business?"

To which the stubborn Ben boomed: "Does Macy tell Gimbel?"

21 January 1941, Washington (DC) Post, pg. 22:
THE "Does Macy's tell Gimbel's?" gag has been a standard one in New York for many years. It was somewhat shattered last night at the Copacabana, when two young couples were seated at a ringside table and conversed at great length. . . ."Well, Macy's may not tell Gimbel's," said one of the young men, "But Bloomingdale's does."

5 June 1944, Chicago (IL) Daily Tribune, pg. 12:
"Would Macy's tell Gimbel's if they snagged a lemon?" snapped Jack.

11 January 1946, New York (NY) Times, pg. 24:
In other words the older circuit will give the Johnny-come-lately as little help and comfort as it can. After all, does Macy's tell Gimbel's?

5 June 1947, New York (NY) Times, pg. 32:
As a lesson in merchandising not only store products but good-will this "Miracle on 34th Street" is a dandy. Does Macy's tell Gimbel's? It should!