Here's how it's often described:
* Mischievous: The person seems to be up to something, perhaps a prank or a harmless trick.
* Playful: The grin suggests a lighthearted and fun-loving attitude.
* Knowing: The person might be privy to some secret information or have a sense of amusement at the situation.
* Slightly wicked: There's a touch of devilishness, but not necessarily malicious.
Think of a mischievous imp or sprite, and you'll get the idea.
Examples of situations where someone might have an impish grin:
* A child who has just gotten away with a small prank.
* A friend who has just told a funny joke and is enjoying the reaction.
* A mischievous character in a story who is about to pull a fast one.
An impish grin is often contagious and can make others feel a little lighter and more playful.