I have been wondering about the term "outsider" art for a long time. I stumble across the term as I look for selling venues for my art but never really knew what it was. Here's a definition I found that seems to match the modern idea of what "outsider" art is. Outsider art is the term commonly used to refer to creative works made by people with no formal art training who create work based on their inner experience, without reference to mainstream art practice.
The piece above was done by a Texas butcher Charles A. A. Dellschau who lived from 1830 to 1923. He had a love of the idea of flight, which showed in his Jules Verne-style flying machines.
Wikipedia has a fascinating page on this topic. Here's to a creative day!