

This appears to be a variation of a quote often attributed to Caskie Stinnett in 1960, "A diplomat...is a person who can tell you to go to hell in such a way that you actually look forward to the trip" https://books.google.co.uk/books?id=kcycAQAAQBAJ&pg=PA90&dq=%22A+diplomat+is+a+person+who+can+tell+you+to+go+to+hell+in+such+a+way+that+you+actually+look+forward+to+the+trip.%22 but which appears to have been in common use in the 1950s and is first recorded in the Seattle Daily Times in 1953 as "Diplomat—one who can tell you to go to hades and make you look forward to the trip". http://www.barrypopik.com/index.php/new_york_city/entry/a_diplomat_is_a_person_who_can_tell_you_to_go_to_hell_so_that_you_look_forw/
Misattributed
Variant: Diplomacy is the art of telling people to go to hell in such a way that they ask for directions