Green’s Dictionary of Slang

1. (orig. Aus.) small, trivial, insignificant, esp. of business conducted from one’s own home.

1910		Bulletin (Sydney) 1 Sept. 11/3: Of course, the habit of being found dead is a thing to be discouraged [...] but if anyone decides that it is really necessary to be found dead, then it is just as well to do it amid a set of curious circumstances as to do it in a sordid, ordinary backyard manner.
1915		Bulletin (Sydney) 1 July 22/2: What sort of backyard bushman is this ‘Harrison O.,’ who snorts with derision [...] at the Rev. Spurr’s story about an Australian girl running a dingo down on horseback?
1966		C. Stead Cotters’ England (1980) 157: It’s all private self-indulgence, back-yard calumny.

2. (US black) surreptitious.

1934

M. Fulcher ‘Believe Me’ in Afro-American (Baltimore, MD) 17 Nov. 12/3: That talk [...] concerning the big politician (large) and the schoolmarm (little) who might be classed as having a backyard romance.