别乱开玩笑，这是个严肃的事。

Stop being facetious; this is serious.

《牛津词典》

“这不是开玩笑，赛赛曼先生。”小姐严肃地回答。

"It is no joke, Mr. Sesemann," replied the lady seriously.

youdao

这话，苏淳曾经跟海萍开过玩笑。海萍严肃地说：“绝对不会。”

Su Chun used to joke about this thought with Haiping but she turned out to be very serious and said "that's never gonna happen."

youdao