To be honest, this is actually a part of the reason why I decided to get a crappy job as a cashier. I was tired of telling people I was an unemployed high school dropout. At one point I was seriously considering going on disability, but it just never happened since I didn't want to go through with getting a psych evaluation done by some doctor who wouldn't believe that there was anything wrong with me. That, and my stupid pride convinced me that it would be better to get a job I could barely handle.
The job I'm at right now is a really poor fit for me, as I have to deal with lots of people, endure lots of stimuli, and conceal a good amount of my anxiety about dealing with things like raw meat drippings, saliva on people's bills, etc. I'm only doing it because the grocery store that hired me was the first (and only) place that was willing to call me back after the initial interview, and I needed the money. My bosses say that I'm doing OK, but I have a feeling they're only saying that to be polite.
Back on topic though, I honestly don't know what a good response would be. I mean, in your situation I would probably say something smart-alecky like "I raise unicorns", or "I'm a corporate slave", though these may not be the best responses if whoever you're talking to is looking for a serious answer.
I think that most of the time, when people ask one another what they do for a living, they're looking for something to talk about, because let's face it; modern North American society is highly career-focused, and people are practically defined by what they do to make money, so most people don't have much to talk about besides work. The only reason why I ever talk about work is because it just feels good to b***h and complain about it and let it off my chest, not because it actually interests me. Whenever I want to actually make conversation, I tend to prefer talking about music, computers, and other topics that actually interest me.