Green’s Dictionary of Slang

(US) pertaining to the upper or upper-middle classes.

1865		G. Hamilton New Atmosphere 32: The brown-stone friends are shocked and scandalized, which is probably the best thing that could happen to them.
1873		Winfield Courier 15 Feb. 1/4: The brownstone members of that institution have pretty generally attempted to keep it a secret [DA].
1888		J. Bryce Amer. Commonwealth II III lxii 443: The so-called ‘brownstone districts’ in New York City have, I believe, good [political] machines [DA].
1902		E. Townsend Chimmie Fadden and Mr Paul 18: I has been waltzing about in brownstone society for years, now, but dere is one ting I can’t get wise on.
1941		D. Burley N.Y. Amsterdam News 1 Feb. 16: Sang Froid was [...] worn with cultured ease by browstone social noblemen and dowager alike.
1960		(ref. to 1870s) M. Berger Meyer Berger’s New York (2004) [ebook] In the Eighteen Seventies the term “brownstone” was more or less interchangeable with ‘blueblood’ in this city [...] flossy clubs had ‘brownstone membership’ and the ‘brownstone vote’ was the silkstocking vote.