He has to visit them on the sly.

他只得偷偷地去看望他们。

《牛津词典》

Under the cover of the smog, He snatched the pot on the sly.

在烟雾的掩护下，他偷偷地拿走了那只壶。

youdao

Some boys smoke on the sly.

有些男孩子暗地吸烟。

《新英汉大辞典》

So they tried to keep up appearances while indulging in a sensual life on the sly.

NEWYORKER: The House Behind a Weeping Cherry

And there are also gifts she gives on the sly that, she says, not even her husband knows about.

FORBES: From the heart

All this work is legal, mind you, but it is usually done on the sly, often without the knowledge of the targeted company.

FORBES: Gotcha: Pushing The Limits of Due Diligence