A stupefier is something that makes you unable to think or feel properly, like a very strong medicine or a shocking event.
Stupefier can make people think of confusion, surprise, or being very tired.
Examples:
The medicine is a stupefier.
The loud noise was a stupefier.
The stupefier made him feel very strange.
The stupefier made her fall asleep in class.
The stupefier was so strong, it made everyone forget what they were doing.
The stupefier was like a big surprise party.