Wiktionary
sharp cookie
n. (context idiomatic English) One who is intelligent, bright, or sharp; especially, one who can identify attempts to deceive or mislead.
Usage examples of "sharp cookie".
He didn't want to disturb anything unnecessarily, either, because the cops undoubtedly had photographs of the scene, and some sharp cookie might notice if anything was moved.
I had a couple of classes with him, and he's a pretty sharp cookie.
Priana was a sharp cookie, chock-full of questions and a lot of fun to be around.
That's why he's employed a sharp cookie like Rickman, and if you think Rickman is acting for you then you're out of your mind.