in the buff

From buff (“bare skin”, literally “brownish yellow”), an allusion to the color of tanned light skin.

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in the buff

1. (idiomatic, informal) Nude.

   She was in the buff on the beach.

   The streaker ran across the playing field in the buff.

   • 2021 October 18, Ian Lecklitzner, “The Inevitable Rise of the Work-From-Home Nudist”, in MEL Magazine‎:

     Not to mention, nudity can be just plain convenient. “Laundry is minimal,” Schulte notes. It also doesn’t hurt that being in the buff spices up his workday.

• Gary Martin (1997–) “In the buff”, in The Phrase Finder.