deceiving people in a clever way in order to get what you want

He's a sly old devil - I wouldn't trust him with my money. 他是个老滑头——我不会把钱交托给他的。

seeming to know secrets

"You'll find out eventually," said Mary, with a sly smile. “你最终会知道的，”玛丽诡秘地笑着说。

If you do something on the sly, you do it secretly because you should not be doing it.

He drives his mother's car on the sly while she's at work. 他在母亲上班的时候偷开她的汽车。