Deceiving people in a clever way in order to get what you want. (Cambridge Dictionary)
Seeming to know secrets. (Cambridge Dictionary)
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. (Collins Dictionary)
If you describe someone as sly, you disapprove of them because they keep their feelings or intentions hidden and are clever at deceiving people. (Collins Dictionary)
Wise in practical affairs. (Merriam – Webster)
Displaying cleverness: Ingenious. (Merriam – Webster)
Clever in concealing one’s aims or ends: Furtive. (Merriam – Webster)
Having or showing a cunning and deceitful nature. (Oxford Dictionaries)
Sly and slick have similar meaning.
Are You Sly as a Fox?