Green’s Dictionary of Slang

1. (US) a soft drink [belly wash n. (1)].

1909		J. Ware Passing Eng. of the Victorian Era 25/2: Belly-washer (Amer. Saloon). Lemonade or aerated water.
1925		Huntingdon Press (IN) 14 Apr. 4/1: Many months ago the department of agriculture issued warnings [...] to avoid some kinds of drinks. The department spoke very frankly of these as ‘belly washers’.
1941		Kingsport Times (TN) 22 July 4/2: Non-alcoholic beverages, the high-hat name for pop, tonic, and assorted belly-washers.
1963		Charlestown Courier (IN) 8 Aug. 4/4: A few belly-washers and a milkshake.
1970		Greenville News (SC) 1 Feb. 45/1: Coach kendall would pass out soft drinks [...] I never had a drink that tatsed so good as those ‘belly washers’.
1982		Greenville News (SC) 22 July 93: [advert] Younguns under 6 kin eat free. They kin git chicken or fish or a peanut butter an jelly Samwich! — An a belly washer.
1998		Albuquerque Jrnl (NM) 23 oct. 5: [advert] Belly Washers Drinks. Gallon. Limit 2, please. $1.

2. wine [belly wash n. (2)].

1966

in DARE.