Green’s Dictionary of Slang

(US sporting) any race or sporting contest in which the result has been fixed in advance; thus used in non-sporting context (see cites 1935, 1951).

1887		Courier-Journal (Louisville, KY) 19 Jan. 5/1: No one calls the affair a hippodrome. The fight was to be for points, and the winner was to get 75 per cent. of the house receipts [DA].
1889		World (N.Y.) 30 Oct. 2/2: ‘Oh, this is a hippodrome,’ shouted a bleacher.
1915		Everybody’s Mag. June 698/1: They bunted us dizzy, ran the bases hog-wild, and turned the game into a hippodrome before the inning was over [DA].
1935		S. Walker Mrs Astor’s Horse 210: When Browning’s separation action came to trial at White Plains [. . .] the public was treated to a hippodrome. Testimony brought out that Browning had given Peaches a honking African gander which he insisted on leading around with a string wherever they went.
1951		R. Starnes Other Body in Grant's Tomb 131: I wrote a couple of takes [...] hinting that his spectacular trip to the gutter was merely hippodrome to cover a painstaking investigation into the secret racket.