Green’s Dictionary of Slang

1. anything that can be accomplished with minimum effort.

1940		W.R. Burnett High Sierra in Four Novels (1984) 331: I wish I had four like you. It [i.e. a robbery] would be a waltz.
1949		W.R. Burnett Asphalt Jungle in Four Novels (1984) 217: Walk to Eddie’s [...] When you get beyond the Market it’s a waltz.
1970		New Yorker 1 Aug. 62: After that, it was a waltz.

2. an easy success; esp. in sporting use, e.g. a boxing match in which neither fighter makes much effort.

1928		P. Gallico ‘The Yellow Twin’ in Goodstone Pulps (1970) 40/1: If [...] you went out there expecting a waltz, and got knocked off, you’d never forgive me.
1942		N. Algren Never Come Morning (1988) 265: ‘Don’t fight boys — we don’t expect it!’ ‘A little music with dat waltz!’.
1947		B. Schulberg Harder They Fall (1971) 123: Was the Commish justified in tying up both those bums’ purse after the waltz last Friday night.
1968		Time 5 July 38: Though Dancer eased him up at the end, Nevele Pride won in a waltz.
1973		C.P. Rosenberg in Heller In This Corner (1974) 87: Every fight in there was a fight. It wasn’t one of them dances or waltzes or anything.