Armatage Shanks song meaning

"When I wrote that song it was right before Dookie came out, and I was really at odds with myself. I was like, man, do I really want to do this? A lot of the time I was thinking about suicide, how it's so easy to kill yourself, but so hard to stay alive. I was in a break-up with my then-girlfriend, a total, raving punk rocker who didn't approve of me being on a major label. She moved down to Ecuador, saying she couldn't live in a world with McDonald's and such. It was fucking me up pretty bad" - Armstrong.

Armatage Shanks is a great introduction to Insomniac, an album that was written at a time when the members of the band were going through some hard confusing times, and is therefore filled with songs about insecurity, panic and doubt. In Armatage Shanks the narrator is saying that he doesn't trust anyone, let alone himself and calls himself a "loner in a claustrophobic mind". He expresses his distrust and irritation by saying "I must insist on being a pessimist". There's no one to trust, nothing makes sense, so being pessimistic is not just a current state of mind - it becomes an ideology.

There is an interesting theory about the name of the song. Armitage Shanks is a British brand of toilets, and it is quite possible that the author took that for a name of the song with a "life going down the drain" implication. In this case, the word "Armitage" would be purposefully misspelled to avoid using the trademark.
There is a legend that Armstrong read the name of the brand in a London toilet and then decided to use it as a song title. Of course, this is only a legend; however, this kind of similarity of names is not very likely to be just a coincidence.