Originally invented by the Inuit, who needed clothing which protected them from windchill and wet, while hunting and kayaking in the arctic region. Underneath the anorak the Inuit wear warm clothes. Inuit anoraks have to be regularly soaked with train oil (fish oil) to keep their water resistance. Today, the jacket is named after it because it attempts to be just as waterproof.
In British slang, anorak has come to mean "geek" or "nerd", originally from their use as the invariable wear of train spotterss, and then by extension to refer to anyone with an unfathomable interest in detailed information regarded as boring by the rest of the population - aided by the intuition that only a geek would wear something so terminally unfashionable.