Idiomatic, Usually, With the The social elite, the highest social class.
I judged from your remark about the diligence and industry of the high Parisian upper crust that it would have some point.1894, Mark Twain, “A Little Note to M. Paul Bourget”, in Essays on Paul Bourget:
Some accents reveal the distinctive bray of the upper crust, but most are generic middle class.2006 June 18, J. F. O. McAllister, “A New Kind of Elite”, in Time:
1 more example
“You ever see the movie Spotlight?” “About the Boston Globe reporters who uncovered the sexual abuse coverup in the Catholic Church?” “Yeah, exactly.” “No.” “Oh. Well, in that I played Ben Bradlee, Jr. And that character begins with the accent, because the real Bradlee is from Boston. But he's an upper-crust guy. [Boston accent] Not like some jamoke standing around Kelly's Tavern in, uh, Revere Beach, you know what I mean?” “Very good, John Slattery.” “[regular accent] Not bad, right? But I was talking to him.”2023 August 10, Sam Johnson & Chris Marcil, “Urgent Care” (11:58 from the start), in What We Do in the Shadows, season 5, episode 6, spoken by John Slattery (John Slattery):
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Used other than figuratively or idiomatically: see upper, crust. The topmost layer of a bread, pastry dish, or other item with a hardened coating.
She went to get her tarts . . . but some one had stolen all the fruit out of them by lifting up the upper crust.1871, Louisa M[ay] Alcott, “chapter 20”, in Little Men: […], Boston, Mass.: Roberts Brothers, →OCLC:
Idiomatic, Informal Posh, upper-class; pertaining to the upper crust.
An upper-crust London party.
They're aiming for a more upper-crust clientele.
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In 1986, Takako Doi was elected leader of the Japan Socialist Party; another woman is the president of Takashimaya, Japan's most upper-crust department store chain.1989, Roy Thomas, Japan: The Blighted Blossom, page 172:
But Prentice had grown up richer and more upper-crust than Richard had any idea of.1993, Alice Adams, Almost Perfect, published 1998, page 192:
Patinkin’s melodious voice is a natural fit for animation, and he breathes just the right type of upper-crust life into Hugh in 22 short minutes.2013 October 13, Erik Adams, “TV: Review: THE SIMPSONS (CLASSIC): “Lisa’s Wedding” (season six, episode 19; originally aired 3/19/1995)”, in The Onion AV Club:
That was another story, and the retirees who frequented the relatively lowbrow kino Ridge were more likely than the more upper-crust folks at Amado or Rio Rico to shut up and pay their fines than they were to hire criminal defense attorneys.2012, J. A. Jance, Left for Dead, page 43: