It's a private matter.

这是件私事儿。

《牛津词典》

It was clear that she wanted to discuss some private matter.

很明显她想讨论一些私事。

《柯林斯英汉双解大词典》

This seemed like a very private matter, but Peter thought about it carefully the whole night.

这似乎是一件非常私人的事情，但彼得整个晚上都在仔细地思考。

youdao

Maya Lin has said that death is a private and personal matter.

VOA: special.2009.05.06

It's a private matter. He decides.

被冒犯的一方，由于是私事，所以由他自己决定。

耶鲁公开课 - 旧约导论课程节选

Augusta's chairman dodged the prickly issue of women's membership Wednesday, saying it was a private matter.

CNN: Augusta National a powerful holdout among men-only clubs

"I think it is a private matter for the Deputy Prime Minister, " he said.

BBC: NEWS | UK | UK Politics | Prescott admits affair with aide

The pursuit of happiness, Lord Layard's book will convince most people, is a private matter.

ECONOMIST: The economics of happiness: Can't buy it? | The