"Two cents plain" in the lower east side of the 1920s and 1930s meant a glass of seltzer water. Harry Golden's 1959 New York memoir of this title helped to popularize the old term.

http://www.urbandictionary.com/define.php?term=a+two+cents+plain&defid=1809688
a two cents plain
A two cents plain is a glass of seltzer water. During the Great Depression, one could get an egg cream for twenty-five cents (seltzer, chocolate syrup, and milk), a chocolate milk (milk and chocolate syrup) for a dime, a chocolate soda (seltzer and chocolate syrup) for a nickel, or plain seltzer water for two cents. Two cents plain.

I'll have an egg cream for my lovely date, and I'll take a two cents plain.

by Judith Chicago Jun 14, 2006

26 July 1959, Chicago Daily Tribune, pg. B3:
FOR 2c PLAIN, by Harry Golden [World, 313 pages, $4]
(...)
Always, like the essayists who have become classic, Golden exploits his own temperament and experience. he remembers his Uncle Sholem, introduces his brother Jack, revisits in memory the lower east side in New York where a large glass of seltzer was ordered by saying, casually, "Give me for two cents plain," followed by a chiseling gambit, "put a little syrup on the top."

19 October 1959, Appleton (WI) Post-Crescent, pg. A3:
(Excerpt from Harry Golden's For 2c Plain -- ed.)
I remembered in this story how the entire east side civilization was addicted to seltzer (carbonated water) and the great variety of sweet drinks mixed with seltzer. You bought a drink from a man behind a marble counter at any of the hundreds of soda-water stands scattered through the section. A small glass cost a penny -- "Give me a small plain." No syrup. Syrup cost another penny. For a large glass, you said, "Give me for two cents plain." As the man filled the glass, you said casually, still holding tightly to your two pennies, "Put a little on the top." You wanted syrup, of course, but you didn't want to pay the third penny. The next time you tried it, though, the man insisted on your two pennies first before he started to fill the glass. "I know my customers," he'd say.

21 December 1983, Washington Post, pg. D2:
"That might have been fine for Florida, but it was not okay for New York City," New York Mayor Ed -- ed.) Koch said. "That was orange juice and what New York needs is seltzer. And this is more than even two cents plain."

1 June 1997, New York Times, "On Language" by William Safire, pg. SM26:
Remember ? It was the sole ingredient of a two cents plain, which is what residents of the Bronx called a glass of that sold for 5 cents in the '40's, when I last ordered one at the High School of Science .The German word comes from mineral water from Nieder Selters, Prussia, called Selterser Wasser.

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