Dr. Pepper began in Waco, Texas. In the 1930s, “Waco” was drug store slang for an order of Dr. Pepper. “Dallas” was slang for an order of Coca-Cola (then bottled in Dallas). “Waxahachie” was slang for an order of one Dr. Pepper and one Coke (Waxahachie is half-way between Waco and Dallas). The “soda jerker” slang died out by the 1950s and 1960s.
   
 
Dr. Pepper website FAQ
Q: How did Dr. Pepper get its name?
A: A pharmacist named Charles Alderton created Dr. Pepper in 1885. Alderton worked at a drugstore in Waco, Texas owned by Wade Morrison. Legend has it that Morrison named it “Dr. Pepper” after the father of a young girl he was once in love with.
   
10 July 1938, Galveston (TX) Daily News, pg. 4, col. 7:
Car-Hop Jargon Complicated, but
If Mastered Is Never Forgotten
And Makes Taking Orders Simple
(...)
Have you never been sitting in your favorite drug store peacefully sipping a tall cool drink or munching an ice cream cone and meditating passively about something far away, only to have your musings harshly interrupted by a voice from the back shouting to another person behind the counter:
 
“Shoot a crowd; bust one; Waco; straw in, all the way; eighty-six!”
 
Certainly you have. But you probably wouldn’t believe that all those words so uttered were orders for:
 
Three cokes, a lemonade, a Dr. Pepper, a strawberry soda with strawberry ice cream, and six glasses of water.
 
In “soda-buster” or “car-hop” parlance, practically every drink and delicacy concocted behind the fountain has its own individual name, shortened usually or made more impressive for purposes of remembering by association with something else.
 
Although some of the downtown drug stores have eliminated the use of such slang by substituting written orders for everything, beach drive-in stands still largely use the slang system for drinks, and some still do for orders of food.
 
Like swimming the car-hops say, once the lingo is mastered, it is never forgotten. About two months are required for complete mastery of the auxiliary language, and after that apprenticeship, any novice car-hop develops all the confidence necessary to bawl out abbreviated English with the best of the veterans.
 
After the car-hop places the order, the person behind the fountain or counter, often called the “soda-buster,” repeats or “echoes” the order to avoid mistakes in filling it.
 
The next time you stop by your favorite drive-in stand or drug store for a drink, have the following dictionary of soda jargon at your elbow. When the car-hops get busy and start shouting their orders at the fountain, don’t be upset or curious. Look up what your neighbor is ordering. And don’t fool yourself. It’s not as easy as it sounds.
 
“Eighty”—a glass of water. Add one for each additional glass. However, “eighty-six” shouted by the man or girl behind the fountain (Col. 8—ed.)  usually means, “We’re out of that. Try something else.” In some places, “eighty-nine” means that an attractive girl has entered the place, and all accordingly turn to look at her.
 
“Choc-in” means chocolate ice cream soda, and “straw-in” and “van-in” mean strawberry or vanilla sodas. Ordinarily vanilla ice cream is used, but “all the way” added to the soda means that ice cream the same flavor as the soda ordered is to be used.
 
“Shot” means a Coca-Cola. For each additional one, a new name is added.  Thus “shoot a pair” is two; “shoot a crowd” is three’ “shoot the navy” is four, and “shoot the army” is five.
 
Dr. Pepper is called “Waco” because the drink, although it is now bottled here, originated in Waco. Thus: “Waco a pair,” “Waco a crowd,” etc.
 
“Cut the hail” means that the drink is to be made without ice. “In the rain” signifies when added to an order for a drink that it is to be made with plain water, instead of the carbonated water usually used.
   
3 September 1975, Dallas Morning News, section D, pg. 1:
Soda jerking,
“bee’s knees”
slang in past
By RENA PEDERSON
You don’t hear many orders for a “Dobbin” or a “Waco” at pharmacies these days.
 
But back in the 1920s and 1930s, when soda fountains were the popular place for everyone to sip and spoon, that’s how a barbecue sandwich and a Dr. Pepper were ordered. Two barbecues with nothing else on them became a “Pair of Dobbins,  hold everything but the heat.”
(...)
“‘Shoot one” was a Coke,” said Mrs. Newton, “and ‘Stretch One’ was a large Coke.”
 
Likewise, Mrs. Sanderson chimed in, “‘Shoot a wild one’ was cherry Coke. ‘Shoot one frowning’ was a lime Coke and a ‘Shanghi’ was an iced tea.”
 
The two said “Squeeze one” meant a lemonade, “Draw one” called for a coffee and “Vanilla Dust” was actually a malt.
 
Charlie Day, who’s been “jerking sodas” at the Highland Park Pharmacy for 51 years, recalled that a “400” used to be universal language for a small milk and an “800” was a large milk, or course.
 
He also explained a Dr. Pepper was called a “Waco” because the drink originated in Waco. “Because they bottled Coca-cola in Dallas, they called a Coke a ‘Dallas.’ If you had an order for one of each, you called the double order a ‘Waxahachie,’ because it was halfway between Dallas and Waco.”