To smile in an unpleasant way, often because you are happy about someone else's bad luck or because you think you are better than someone else.
Smirking can make people feel like the person is mean or not very nice.
Examples:
He smirked when I dropped my ice cream.
She smirked at her rival's mistake.
The cat smirked at the mouse.
The little boy smirked when his sister fell down.
The villain smirked as he watched the hero fail.
The mean girl smirked at everyone in the room.