Green’s Dictionary of Slang

(US) respectable fools, considered as a class in their own right; a synon. for the modern ‘Middle America’.

1936		Mencken Amer. Lang. (4th edn) 560: [note] I have never devised anything properly describable as slang, save maybe booboisie. This was a deliberate invention. One evening in February, 1922, Ernest Boyd and I were the guests of Harry C. Black at his home in Baltimore. We fell to talking of the paucity of words to describe the victims of the Depression then current, and decided to remedy it. So we put together a list of about fifty terms, and on Feb. 15 I published it in the Baltimore Evening Sun. It included boobariat, booberati, boobarian, boobomaniac, boobuli and booboisie. Only booboisie, which happened to be one of my contributions, caught on.
1977		Maledicta 1:2 135: Philistine was introduced by Thomas Carlyle and popularized by Matthew Arnold as a term for a crass, materialistic, anti-intellectual member of what H. L. Mencken called the booboisie.