“Diplomat—one who can tell you to go to hades and make you look forward to the trip” has been cited in print since at least 1953 and is of unknown authorship. Caskie Stinnett (1919-1998) is often credited for originating the saying in his book Out of the Red (1960), but there are several earlier printed citations.
 
Green Bay Packers football coach Vince Lombardi (1913-1970) was described by his players in similar “diplomatic” language: “When Lombardi tells you to go to hell, you look forward to the trip.”
   
 
Wikiquote: Diplomacy
Diplomacy is the art and practice of conducting negotiations between representatives of groups or nations. In an informal or social sense, diplomacy is the employment of tact to gain strategic advantage, one set of tools being the phrasing of statements in a non-confrontational, or social manner.
 
Unsourced
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.
. Caskie Stinnett
       
24 June 1953, Seattle (WA) Daily Times, “Hits and Misses,” pg. 6, col. 5:
Diplomat—one who can tell you to go to hades and make you look forward to the trip.—Everson News.
 
Google Books
Railway Carmen’s Journal
Brotherhood Railway Carmen of the United States and Canada, Brotherhood Railway Carmen of America
Volume 60
1955
Pg. 151:
“A true diplomat is a gentleman who can tell you to go to hell in such a nice way that you look forward to the trip.”
 
Google Books
The Student Lawyer:
Journal of the American Law Student Association
Volume 1
1955 (WorldCat shows at 1957 date for Volume 1—ed.)
Pg. 13:
You know what a diplomat is— he is a fellow who can tell you to go to hell, but he does it in such a nice way that actually you look forward to the trip.
 
Google Books
The Australian Law Journal
Volume 31
1957
Pg. 289:
Probably you know the definition of a diplomat as a man who can tell you to go to Hell in such a nice way that you actually look forward to the trip.
 
27 November 1959, Galveston (TX) Daily News, pg. 4, col. 1:
A diplomat can tell you where to go and make you look forward to the trip.
       
OCLC WorldCat record
Out of the Red.
Author: Caskie Stinnett
Publisher: New York, Random House [1960]
Edition/Format:  Book : Fiction : English
 
Google Books
7 December 1962, Life magazine, “The Miracle Maker of Green Bay, Wis.” by Marshall Smith, pg. 52, col. 4:
As one Packer player puts it, “When Lombardi tells you to go to hell, you look forward to the trip.”
   
24 December 1965, Baltimore (MD) Sun, “Lombardi Called Very Fair, ‘Treats All Of Us Like Dogs’” by Milton Gross, pg. B3:
Signs On The Wall: “Lombardi works you so hard,” said one of them, “that when he tells you to go to hell, you look forward to the trip.”
   
Google Books
The Speaker’s Handy Reference
By Edward L. Friedman
New York, NY: Harper & Row
1966
Pg. 119:
The most important quality or attribute for a successful politician is tact. Some politicians have so much tact that they can tell you to go to hell in such a nice way that you will look forward to the trip.
 
Google Books
The Modern Handbook of Humor
By Ralph Louis Woods
New York, NY: McGraw-Hill
1967
Pg. 142:
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.
— Caskie Stinnett, Out of the Red, 1960
     
Google News Archive
4 january 1968, Reading (PA) Eagle, “Quotations Old and New,” pg. 18, col. 4:
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.—Caskie Stinnett.
   
Google Books
1,001 logical laws, accurate axioms, profound principles, trusty truisms, homey homilies, colorful corollaries, quotable quotes, and rambunctious ruminations for all walks of life
By John Peers and Gordon Bennett
Garden City, NY: Doubleday
1979
Pg. 97:
Robertson’s Rule: A diplomat is someone who can tell you to go to hell in such a way that you will look forward to the trip.
   
Google Books
Diplomat’s Dictionary
By Charles W. Freeman, Jr.
Diane Publishing Company,
1994
Pg. 112:
“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.” Caskie Stinnett, 1960