Green’s Dictionary of Slang

orig. horse-racing, ‘a smooth and easy victory, passage, or accomplishment’ (OED); in Aus. prison use, a jail sentence that offers no problems.

[1895		Sport (Dublin) 26 Oct. 5/7: Croaghpatrick [...] simply made hacks of his opponents [...] e in the National Hunt Flat Race, and gave his owner a perfect armchair ride].
1984		B. Ellem Doing Time app. C 239: He got the greatest armchair ride I’ve ever seen in the place. He only served about eighteen months in jail.