I was kind of expecting that question after rereading my post. It's so ingrained in my vocabulary with some of my friends that I forget it isn't familiar to most. It's an inside joke that has spread beyond the state's borders with those that commonly use it. It seems to take anywhere it's used. I know it's commonly used in Seattle, Alaska, Colorado, and Jersey at this point. No offense to anyone named Jim.
It was the name of the most ridiculous tourist I've ever seen. My cousin and I were in Yosemite at the top of Glacier Point when we met "Jim" who proceeded to ask many really smart questions such as "Is that Half Dome?" when pointing to the biggest slab of granite in the valley that's shaped like a dome that's been cut in half. He then took photos of his 5 year old son as he hucked rocks off the 1000' cliff down into the valley where people are walking around. It went on and on with this guy so we started telling each other the rest of the weekend not to be a "Jim." It became "What a Jim", etc etc. and just spread from there.