"sly smirk" is a perfectly valid and grammatically correct part of a sentence in written English.
You can use it to describe a facial expression when someone has just said or done something and they are aware that they have gotten away with it. For example, "John's teacher gave him a stern look, but John responded with a sly smirk, as if he knew he had gotten away with something."
I see that sly smirk you make as you wrap your brain around all the other things you might be wrapped around if you keep gazing in my direction.
It was another moment in which he shifted gears again, his signature sly smirk turning plaintive as he said in a sincere and hushed tone, "Money is a bad friend.
With his darting rat eyes, raspy voice, sly smirk and street hustler's swagger, Jack revels in the fact that he is bad news.
"So, what's next?" Jen asked with a sly smirk.
She gave me a sly smirk and opened the door into her little red world.
Fallon wrote, "Chenoweth's every sly smirk, eyebrow tick, and note sung well, that was my week's highlight".
There's something so knowing in Simon's direct address to us – Boyle keeps cutting back from the action to McAvoy's sly smirks – that your defences, and hackles, will instantly rise if you're at all averse to being winked at.
"Mitch," he said, "I know that behind those sly smirks and sad eyes thrown over your shoulder lies a need to dominate, at least a little... and I think we all know that, for all his bluster, Harry just wants to be told what to do.
She's wonderfully mature, mixing some of Erin Brockovich's been-there-done-that attitude with a sly "Pretty Woman" smirk.
Smallweed replies with a sly little smirk: I'm so glad that you asked me that question.
When to shudder and when to smirk: as you watch this sly marital fable of secrets, lies and homicidal intent, you're never sure of what your reaction should be.
