When a 'y' is added, it becomes 'impy', which means that one would look, or act particulary bad, or like a monster.
Impy is often used as a codename between friends to describe or discuss another person undetected and without their knowledge.
2. That teacher is an imp.
3. You look very impy today.
4. You are acting very impy today.
5. isn't Impy so wierd?