Keep your shirt on! It was only a joke.

别生气！开个玩笑而已。

《牛津词典》

"It was only a joke," he roared, while she comforted her boys, and Wendy hugged Nana.

“这只是个玩笑。”他咆哮说，而她安慰她的孩子们，温迪拥抱了娜娜。

youdao

That's only a joke.

那只不过是句玩笑话。

《新英汉大辞典》

Recently Brass Eye offended the sensibility of huge numbers of people and pleaded, "it was only a joke".

BBC: Religious satire: Is it beyond a joke?

So it's only a joke.

BBC: Take your politics too seriously

It was grim medicine, sweetened by only half a joke.

ECONOMIST: It might win Gordon Brown an election. But then what?