tall order

Example: “Feeding forty people from one small kitchen is a tall order.”

Origin: American English, documented in print by 1893, in Francis Adams's The New Egypt: A Social Sketch. "Tall" carried a 19th-century American slang sense of exaggerated or excessive — a usage that survives today mainly in "tall tale" — while "order" drew on its everyday commercial meaning of a customer's specific request or demand. A genuinely large or unusually complicated order placed with a shop or business, one that strained what the business could realistically fulfill, gave the phrase its original, literal commercial grounding before it broadened into its now-general sense: any request, task, or expectation that pushes right up against the limits of what's reasonably achievable.