does Macy's tell Gimbel's

Etymology
and (without the apostrophe) 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

 * 1)  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.