Green’s Dictionary of Slang

(orig. US) an impressive performance, example or instance, both lit. and fig., used of things or individuals; also attrib.

1947		E. Condon We Called It Music 187: ‘Are you sure you know what you’re doing?’ I asked. ‘Is this the kind of music you want for your class act?’.
1977		C. McFadden Serial 53: She had a class act going there.
1983		A-Team Storybook 33: That stunt you pulled [...] was a real class act.
1989		R. McDonald Rough Wallaby 126: Tall skinny weed of a bloke? [...] Married to a classer?
2000		J.J. Connolly Layer Cake 203: He was a class act, shrewd, charming.
2007		P. O’Keeffe Hidden Soldier 275: T]he Hart Group [...] were fantastic and simply couldn’t do enough to try and help me. From start to finish, they were a class act to work for .
2016		Jeezy annotation to ‘Let Em Know’ on genius.com I fuck with people that are genuine [...] Like, genuine motherfuckers. Class act people.