it's no joke; gimme a break!
seriously; no kidding; joking aside; in all seriousness
half joking; being (only) half serious; (in a) jocular vein
(talking in a) half-joking manner
being somewhat (half) joking; half seriously; half in jest; in a jocular vein
jokingly; in the form of a joke
Ein Musikalischer Spaß (Mozart); A Musical Joke