The word is borrowed into English from Japanese. In that language, otaku means roughly the same thing as geek; to convey the English meaning, one would have to say anime otaku or manga otaku. Japanese culture has many other varieties of otaku, such as the pasocon otaku (personal computer geek).
The word is derived from the Japanese o (honorific) + taku (house, home). It was used in the 1980s as a second person singular pronoun among hobbyist photographers. As it gained familiarity, the word was soon used by others to refer to hobbyist photographers. Since the photographers were seen as socially unskillful, reclusive, and obsessed with their hobby, otaku picked up those negative connotations and eventually was used to refer to any reclusive, obsessive hobbyist. Otaku has stronger negative connotations in Japanese than it does in English, although it can be used either positively or negatively in either language.
Examples of loan words include:
wikipedia.org dumped 2003-03-17 with terodump