Green’s Dictionary of Slang

1. used as an all-purpose substitute for a word or phr. one does not wish to use properly.

1929		M. Taft Blues Lyric Poetry (1983) 30: Then I got put out of church Because I talk about diddie wa diddie too much.
1942		Amer. Mercury 55.223. 91: I’d walk clear to Diddy-Wah-Diddy to get a chance to speak to a pretty lil’ ground-angel like that. [Ibid.] 94: Diddy-wah-diddy-a far place, a measure of distance. (2) another suburb of Hell, built since way before Hell wasn’t no bigger than Baltimore. The folks in Hell go there for a big time.
1944		Andy Razaf ‘That’s What I Like About the South’ 🎵 Let me tell ya’ ’bout a place called Doo-Wah-Diddy / It ain’t a town and it ain’t a city / But it’s awful nice and it’s awful pretty.
1960		V. Williams Walk Egypt 114: Diddy-Wah-Diddy where the roofs were made of pancakes and roast ducks flew by with knives and forks in their backs, quacking, ‘Sweet, sweet, come eat.’.
1981		NADS letters n.p.: Doo-Wah-Diddy [...] is often used as a thingamabob word for a place; it occurs in teasing answers to questions about location and so on.

2. an imaginary place, a very distant place, a place one dislikes.

1942

Z.N. Hurston ‘Story in Harlem Sl.’ in Novels and Stories (1995) 1005: I’d walk clear to Diddy-Wah-Diddy to get a chance to speak to a pretty lil’ ground-angel like that.