The deerstalker can be defined in two ways. Firstly, as a man who stalks deer1. A deer stalker in this sense is always a man as women are far too sensible. The second definition is a hat worn by a man while he stalks the aforementioned deer.
The deerstalker hat is usually made of a checked material and is probably most recognisable as the hat worn by Sherlock Holmes, although it must be stressed that there is no evidence to suggest that Holmes himself ever engaged in the act of stalking deer. The most noticeable feature of this hat are the two pieces of fabric that can be folded down around the head to keep the ears warm and which make the voice louder2. Another of the less obvious consequences of the deerstalker, is that if you wear it in the folded-down position, it makes you look like a compete prat.
This has led some parties to speculate that the reason the deerstalker is so used by... er... deer stalkers is because of the hat's ability to guarantee the wearer instant entry in to the official list detailing the 100 Most Ridiculed People on the Planet.