A sly look, expression, or remark shows that you know something that other people do not know or that was meant to be a secret.
His lips were spread in a sly smile.
他心照不宣地咧嘴一笑。
If you describe someone as sly, you disapprove of them because they keep their feelings or intentions hidden and are clever at deceiving people.
She is devious and sly and manipulative.
她阴险、狡诈,而且爱操纵他人。