Green’s Dictionary of Slang

merry-go-round n.²

1. a racetrack.

1903		Ade People You Know 110: He might be out at the Merry-Go-Round.
1919		T.A. Dorgan in Zwilling TAD Lex. (1993) 56: We’ll invite Charlie — He won a chunk at the merry-go-round today.
1935		A.J. Pollock Und. Speaks.

2. a roulette wheel.

1949

Monteleone Criminal Sl. (rev. edn).

In phrases

one the merry-go-round (adj.)

(US) leading a sexually active life.

1943

W.R. Burnett Nobody Lives for Ever 23: ‘A guy with a reputation like yours—well, you don’t expect him to go berserk over a babe like Tony. She’s been on the merry-go-round for some time’ .