This is a serious matter; let's not joke about it.

这是正经事， 可别打哈哈。

《新英汉大辞典》

It is a real deal, not joke.

这是真的交易，不是开玩笑。

youdao

You should not joke about his mistake.

你不应该取笑他的错误。

youdao

But it was not a joke. NASA found seven normal, everyday supermen in a group of five hundred candidates.

VOA: special.2009.06.10

That seems like a bad joke,but if the body theory is right, it's not a joke at all.

又是一个冷笑话，但如果肉体主义者是对的，这就不是开玩笑了。

耶鲁公开课 - 死亡课程节选

Unfortunately, it didn't work very well because people started bickering in his town and the New Harmony became kind of a joke-- it was not a harmonious community.

但不幸的是，这座村落发展得并不顺利,矛盾争执丛生,新和谐村成了笑谈-,因为已经无谓和谐了。

耶鲁公开课 - 金融市场课程节选

That is, that the joke is one to be shared with said editors and the readers, not a joke upon the editors and the readers.

FORBES: Best Letter to the Editor of the Year

It left me suspended in a bad joke in which a living man inexplicably becomes two pints of dust and everyone acts as if this were not a joke at all but, rather, the most reasonable thing in the world.

NEWYORKER: Dead Man Laughing

But given that Casey lives in Arizona and has an American girlfriend and coach, this was clearly a joke, perhaps not the funniest joke, but worth nothing more than raised eyebrows from the American camp.

BBC: Candid Casey eyes fresh start