JD (chocolate milk)

“J. D.” was soda jerk slang for a chocolate milk. The “J. D.” comes from American industrialist John D. Rockefeller (1839-1937), who was said to have liked to drink chocolate milk. Also, chocolate milk cost ten cents, and John D. Rockefeller was known for handing out dimes.
 
“‘J. D.’ (chocolate milk) rises from the fact that this is one of the most popular 10 cent drinks, and the connection of the dime with John D. Rockefeller is obvious” was printed in The Evening Reporter-Star (Orlando, FL) on September 21, 1933. The term “J. D.”—like much of soda jerk slang—is not used today and is of historical interest.
 
     
Wikipedia: John D. Rockefeller
John Davison Rockefeller Sr. (July 8, 1839 – May 23, 1937) was an American oil industry business magnate, industrialist, and philanthropist. He is widely considered the wealthiest American of all time, and the richest person in modern history.
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Rockefeller became well known in his later life for the practice of giving dimes to adults and nickels to children wherever he went. He even gave dimes as a playful gesture to wealthy men, such as tire mogul Harvey Firestone.
   
21 September 1933, The Evening Reporter-Star (Orlando, FL), “Grist—Lingo,” pg. 12, col. 1:
Curious, one day, I asked SAMMY BETHEA at McElroy’s to translate. There’s a pretty good reason for most of the expressions. “J. D.” (chocolate milk) rises from the fact that this is one of the most popular 10 cent drinks, and the connection of the dime with John D. Rockefeller is obvious.
   
27 February 1934, Casper (WY) Tribune-Herald, “Students Give New Meaning to the Alphabet,” pg. 6, col. 7:
LOS ANGELES, Feb. 27.—(AP)—(...) It’s the new vernacular of the work-your-way students who wait on the trade at the Westwood cafeteria of the University of California at Los Angeles and who unofficially constitute the They-Don’t-Speak-Our-Language-Association.
 
The waiter’s shout of “JD” means Jersey dark—a chocolate milk shake to you. “OJ” is orange juice, “SO” limeade (squeeze one), and “CO” lemonade (cut one). “GS” is a peanut butter sandwich (goober san), and “AC” American cheese sandwich.
 
24 June 1934, St. Petersburg (FL) Times, “‘Hay In’ Is Strawberry Soda In Esperanto of Soda Fountains,” sec. 2, pg. 10, cols. 2-3:
A “J.D.” is a plain chocolate milk.
 
3 September 1939, The Sunday Sentinel-Star (Orlando, FL), “Inside Dope on Lake County” by Ormond Powers, pg. 17, col. 5:
The lingo of the drug store boys has always seemed to us nothing short of marvelous. With the help of Guy Neal of Leesburg, who seems to be perfectly at home with simple syrup and cracked ice, we have prepared a list so you can marvel, too.
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A chocolate milk is a “J. D.” (after John D. Rockefeller, who drank a lot of them). Also called “21, 22, 23, etc.,” depending on number wanted.
     
17 April 1949, Sunday News-Democrat (Tallahassee, FL), “Jaunty Jargon of Soda Fountain Meaningful: ‘Jerks” Have a Word For It—or Number!” by Steve Yates, sec. 2, pg. 9, col. 2:
JD or van shake—Chocolate or vanilla milk shake.
 
22 April 1956, Shreveport (LA) Times,  “Soda Jerk Jargon—A Dead Slanguage?” by Dewey Finley, pg. 3-F, col. 4:
GLOSSARY
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JD—Milk chocolate with dip of ice cream.
 
9 August 1958, St. Petersburg (FL) Times, “Have A Lingo All Their Own” by Nancy Osgood, pg. 4-B, col. 7:
ALSO—‘shot” for small Coke—“sweetie” for sweet milk—“churn” for buttermilk—“J-D” for chocolate milk—while a small glass of orange or tomato juice becomes “a stubby O-J” or “a stubby T-J.”
 
15 March 1982, Morning Advocate (Baton Rouge, LA), “Smiley Anders’ Baton Rouge,” pg. 1-B, col. 1:
Eddie Bauer worked behind the fountain, and Prescott (Murphy—ed.) still remembers some of the drug-store slang they used at the time. “A ham sandwich was a ‘grunt,’” he says, “a Coke was a ‘shot,’ a root beer a ‘draw,’ chocolate root beer (ugh!) was a ‘muddy’ and a chocolate milk was ‘J.D.’—because John D. Rockefeller was fond of it.”