The term "milkshake" first appeared in print in 1885. In this context, it was referring to an alcoholic beverage, not the sweet, creamy, non-alcoholic drinks we know today. This alcoholic drink was described as healthful and sturdy and included, among other ingredients, whiskey and eggs.
By 1900, the milkshake better resembled the drink we know today. It was made of chocolate, vanilla or strawberry syrups. It really became popular in 1922 when Walgreens employee Ivar "Pop" Coulson took a "malted milk" drink (milk, chocolate and malt) and added ice cream to it.
The drink became very popular very quickly and was soon being requested by young adults across the country. With the invention of freon-cooled refrigerators in the 1930s, milkshakes making was automated. And then in the 1950s, salesman Ray Kroc bought exclusive rights to a milkshake maker. This machine sped up the production process in major fast food chains like the ones we have today.