Frank Sinatra (1915-1998) and Dean Martin (1917-1995) were two members of the “Rat Pack” who performed in Las Vegas in the 1960s. One comedy bit had one ask the other, “How do you make a fruit cordial?”

“Be nice to him!” was the joke reply. It’s not known exactly when the joke was first told.


Wiktionary: cordial
Adjective
cordial (comparative more cordial, superlative most cordial)
1. Hearty; sincere; warm; affectionate.
2. Said of someone radiating warmth and friendliness; genial.
3. (rare) Tending to revive, cheer, or invigorate; giving strength or spirits.
4. (obsolete) Proceeding from the heart.

Noun
cordial (plural cordials)
1. (UK, Australia, New Zealand) A concentrated noncarbonated soft drink which is diluted with water before drinking.
2. (UK, Australia, New Zealand) An individual serving of such a diluted drink.
3. A pleasant-tasting medicine.
4. A liqueur prepared using the infusion process.
5. A candy (or bonbon) usually made of milk chocolate, filled with small fruits (often maraschino cherries) and syrup or fondant.

Wikiquote: Talk:Frank Sinatra
Dean, How do you make a fruit cordial?
. To Dean Martin during a concert. Martin’s response : “Be nice to him I guess.”
. (Also quoted as: “I don’t know… Tell him a joke?")

Google Books
Vanity Fair
Volume 58
1995
Pg. 102:
“Frank, do you know how to make a fruit cordial?” he asked.

“No. Dean,” answered Frank. “How do you make a fruit cordial?”

Dean took a beat and shrugged. “Why,” he said, “be nice to him!”

Google Books
Rat Pack Confidential:
Frank, Dean, Sammy, Peter, Joey and the Last Great Showbiz Party
By Shawn Levy
New York, NY: Doubleday
1998
Pg. ?:
This night they reverse the lines: Dean works straight to Frank’s quips.

Frank: “Say, Dean, do you know how to make a fruit cordial?”

“No, Frank, how?”

“Why, just be nice to him.”

Polite laugh.

The Internet Movie Database
Quotes for
Dean Martin (Character)
The Rat Pack (1998) (TV)
Dean Martin: Hey Frank, how do you make a fruit cordial?
Frank Sinatra: I don’t know Dean, how do you make a fruit cordial?
Dean Martin: Be nice to it.

Twitter
Suzie Dooré
‏@suziedoore
Jubilee singalong: My Old Man’s a Dustman contains the line “How do you make a fruit cordial? [camp voice] Be nice to him.”
12:53 PM - 3 Jun 2012

SteynOnline
Celebrate Conformity!
by Mark Steyn • Apr 4, 2014 at 10:58 am
(...)
Second, Frank Sinatra and Dean Martin on stage in Vegas throughout the Sixties and Seventies:

Frank: “How do you make a fruit cordial?”

Dino: “Be nice to him.”