"City juice" and "one of the city" is old soda jerk slang for simple "water." The terms were briefly in use in the 1920s and 1930s, but have long since disappeared.

1 March 1936, New York Times, "Lexicon of the Soda Jerker" by Helen Dallis, pg. X10:
Most of the expressions were discovered by Columbia University students who were assigned to cover drug stores throughout the city. Almost all of the terms were found in New York. A few were in use generally, but most of them were limited to certain sections. Water, which might be "aqua pura" in a drug store on Park Avenue, was just "one on the city" down around the Battery.

9 September 1936, Middletown (NY) Times Herald, "Lunch Room Jargon," pg. 4, col. 3:
Water:
Aqua Pura
Potomac's Phosphate
H2O Cocktail
One on the city
Hudson River ale
One off the roof
City juice

7 May 1950, Oakland (CA) Tribune, pg. 122(?):
For months, while waiting for a break on Broadway, Lucille (Ball -- ed.) was a soda jockey in a Times Square drugstore. She still dots her conversation with expressions like: "Straight Kelly" (orange juice), "City Juice" (water) and "Sweet Alice" (milk).

25 June 1978, Chicago Tribune, pg. H32:
Water was city juice, Adam's ale, dog soup, eighty-one or one on the city.