(In the UK) A fan of pirate radio, especially offshore pirate radio.
So called because, while visiting the pirate radio ships, they were known for wearing anorak coats due to the cold and windy climate of the North Sea.
That guy's radio's always tuned to some pirate radio station or other. Even used to visit Laser 558 and Radio Caroline when they were on the North Sea.
Someone with an Expertise in a very niche subject, so you could call someone who has an expertise in the Urartu Civilisation is an Anorak. the word comes for the Greenlandic word for a coat "Anoraq"