De rigueur is a formal adjective that describes things that are necessary if you want to be fashionable, popular, socially acceptable, etc.—in other words, things required by fashion, etiquette, or custom.

// Dark sunglasses are de rigueur these days among fashionistas.

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“Summer swimwear has come a long way since itty-bitty string bikinis were de rigueur for the beach and by the pool.” — Amanda Randone, Refinery29.com, 31 May 2024

It takes a lot of work to be cool. One needs to wear the right clothes, understand the right pop culture references, and use the right lingo before it ceases to be, ahem, on fleek. Rigor is required, is what we’re saying—a strict precision in adhering to the dictates of fashion. Such rigor is at the crux of the adjective de rigueur, a direct borrowing from French where it means “out of strictness” or “according to strict etiquette.” Rigor is also what distinguishes de rigueur from a similar French borrowing, du jour. While the latter describes things that are popular, fashionable, or prominent at a particular time, as in “topic/style/buzzword du jour,” de rigueur describes that which is considered mandatory by fashion, etiquette, or custom for acceptability within a given social sphere or context. A bucket hat, for example, may be the chapeau du jour if it is currently popular or prevalent, but it would only be de rigueur if, among a certain crowd, you would be given the side-eye for not wearing one.