From Team Fortress Wiki
You are... schtupid!
— The Medic on superior intelligence
The Einstein is a headwear item for the Medic. It is a white wig resembling famous scientist Albert Einstein's hair.
This item can only be worn around Halloween or during a Full Moon; at other times of the year, it does not appear in gameplay, unless the server has Halloween mode enabled. However, it can be equipped at any time and can still be viewed on the loadout screen even when it is not visible during games.
October 27, 2011 Patch (Very Scary Halloween Special)
- The Einstein was added to the game.