Green’s Dictionary of Slang

1. a soft drink.

1877		Arizona Citizen (Tucson, AZ) 24 Mar. 3/2: Belly-wash and chewing gum [...] and a host of good things.
1890		Ottowa free Trader (IL) 17 May 7/2: If Bro. Sapp would [...] but sweeten hos own stomach with some kind of ‘belly-wash’, he would not dish up so much slush [...] to his readers.
1893		Record Union (Sacramento, CA) 26 Oct. 4/3: He called for a drink of belly-wash (which is the vulgar term [...] used to designate temperance drinks).
1899		Century Dict.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 22: belly-wash, n. Any soft drink.
1908		Sun (NY) 24 Sept. 6/5: A weak-kneed generation that feeds on bran, nuts and [...] ‘belly-wash’.
1915		County Record (Kingstree, SC) 1 Apr. 2/2: These automobile trucks laden with belly-wash, also known as soda-water.
1917		E. O’Neill Long Voyage Home (1923) 24: Dat’s why I don’t drink noting to-night but dis—belly-wash!
1927		AS III:1 25: All soda water, coca cola, and near beer are called ‘belly wash’.
1959		A. Salkey Quality of Violence (1978) 28: All you get is a dirty glass of belly-wash lemonade mix up with coolie-foot sugar.
1960, 1975		Wentworth & Flexner DAS 30/2: belly-wash Almost any commercially prepared beverage or other drinkable liquid; whisky, beer, coffee, tea, soft drinks, or soup.
1971		Winick & Kinsie Lively Commerce 171: Many B-girls ask for ‘sloe gin,’ which is a signal to the bartender that they want some colored water (‘belly wash’).

2. a weak or bad alcoholic drink.

1894		Guthrie Dly Leader 28 Mar. 2/4: The big burly coons [...] repaired to McCord’s slush stand and drank copious draughts of gullet-birning belly-wash.
1901		Dly Jrnl (Salem, OR) 13 Aug. 2/2: They rallied round the lunch-pile and [...] free guzzle and belly-wash.
1932		W.R. Burnett Giant Swing 137: ‘What’s the matter with him?’ ‘Oh, he drinks too much of Moon Bettner’s belly-wash’.
1939		Howsley Argot: Dict. of Und. Sl.
1983		T. White Catch a Fire 240: A high grade of bellywash like Charley’s Old Jamaican Rum.

3. soup.

1936		H.W. Bentley ‘Linguistic Concoctions of the Soda Jerker’ in AS XI:1 42: BELLYWASH. Soup.
1941		J. Smiley Hash House Lingo 6: Unappetizing, even revolting terms applied [...] bellywash.

4. (US) nonsense.

1895		Guthrie Dly Leader 18 Apr. 1/2: This is business talk from a business man, not mere palaver and belly-wash.
1958		McCulloch Woods Words n.p.: belly wash [...] Another word for baloney.