Green’s Dictionary of Slang

flounder (and dab) n.

a taxi-cab.

1857		‘Ducange Anglicus’ Vulgar Tongue.
1864, 1867, 1870		Hotten Sl. Dict.
1883		N. Devon Jrnl 8 Feb. 7/2: [from The Echo] Call a flounder and dab with a tidy Charing Cross.
1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 71: FLOUNDER: Sydney cab-drivers’ slang a hansom cab of the old variety in contradistinction to the new or brougham cab.
1908		‘Doss Chiderdoss’ ‘Odd or Even?’ Sporting Times 26 Sept. 1/3: First we all take our pitch, and when a ‘flounder’ comes our way, / Each bloke backs ’is luck at guessin’ and ’is pieces ’e will play / On the number of the ‘flounder,’ odd or even.
1915		W. Gippslang Gaz. (Vic.) 10 Aug. 3/6: A cab is a flounder and dab: abbreviated, a flounder.
1935		A.J. Pollock Und. Speaks 41/1: Flounder and dab, a cab.
1938		F.D. Sharpe Sharpe of the Flying Squad 330: flounder (Flounder and Dab) : A cab.
1944		Maurer & Baker ‘“Aus.” Rhyming Argot’ in AS XIX:3.
1953		Berrey & Van den Bark Amer. Thes. Sl. (2nd edn).
1970		‘Metropolitan Police Sl.’ in P. Laurie Scotland Yard (1972) 323: flounder, a: a taxi.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.