Sly (adjective)

1. Clever or cunning, especially in a deceptive or underhanded way.
2. Showing a willingness to take advantage of opportunities; shrewd.

Origin:

From the old english word "slic" meaning "clever, cunning".

Examples:

1. He had a sly grin on his face.
2. She was too sly for him to catch.
3. The sly fox outsmarted the hunters.
4. He was known for his sly tactics.
5. The sly salesman tried to cheat us.