The Prussian Pickelhaube was one of the first hats made available for the Medic, during the Sniper vs. Spy update for Team Fortress 2.
The hat is team-colored, but can be painted to remain a fixed color if the player has a Paint Can. The design for the Pickelhaube is an enduring stereotype about the First World War. In most Allied propaganda from the war, the Central Powers were depicted wearing Pickelhaube hats. Even in the present day, the hat is seen as a stereotype of World War I. The name comes from antiquated German, when the term 'Pickel' meant a point. In modern German it means a spot (skin blemish), while Haube is an older term for a bonnet or general headgear.
The hat description was contributed by This_is_not_Spy during the Hat Describing Contest. It reads:
A relic from one war brought into another. Your enemy will get the point.
This is not the only historical hat to exist in the game. The Proof of Purchase is another WWI-themed hat, though was typically seen as the headgear of the British Army during the war. In terms of WWII-era hats, the Stahlhelm, Surgeon's Stahlhelm, Pilotka and Officer's Ushanka are all reflective of the German and Soviet armies of the period.
However, these hats are not nearly as common as the Pickelhaube, for it can be purchased for £0.59 from the Mann Co. Store, found via drops, or from Mann Co. Crates (Series 11 crates only).
While a common sight, the Prussian Pickelhaube is no longer available to buy from the Mann Co. Store, and it does not drop any longer. The hat was "discontinued" by Valve alongside a number of others, meaning it can only be found in crates now. This change makes the Pickelhaube one of the first hats discontinued.