The word 'otaku' is from Japanese language which means a homebody who often with an obsession about something (usually anime, manga, games or other indoor hobbies). The original meaning of otaku means 'home' or 'you', written as お宅 in Japanese. But later, the meaning extended to its current modern slang form and written in hiragana おたく or katagana オタク to be distinguished from the original meaning.
In Japanese language, the word often carries negative meaning, but when used in a specific context, it can be an equivalent to 'geek', most of time it means an anime or manga fan, not necessarily with social ineptness.
In Chinese language, the word 宅, which is a valid Chinese character, is taken directly to mean the otaku-ness, or as an adjective of being an otaku (despite its original meaning in Chinese language which also means home). A male otaku in Chinese is called a 宅男, whereas the female equivalent is 腐女 (lit. rotten female).
Despite its loan nature from Japanese language, the word 宅男, when being used in Chinese language, is generally understood as a homebody, without the specific obsession part in the more often usage of otaku.
This song with the title '宅' by Leo Ku uses a very smart play of the word. The theme of the song is about a lonely guy who became a homebody due to being single (implying that he ended a relationship). The lyricist uses the word 宅 to replace all occurrences of 在 in this song for their similar pronunciation, and verbalises the word 宅, effectively emphasises the theme and the title of the song. Creative. I haven't seen this kind of wordplay in other songs.
That being said, I really like this song for its creative value, and also the relevance of its lyrics to me.
习惯了 就这样 静静的