Its what people who think they're clever use as a word for people who are autistic. The "joke" is that autistic sounds like artistic, thus calling them painters like a coward.
Painters breath fumes all day, not that they are all that dangerous or anything, but leads to stereotypes. It’s also an inside joke among constructions workers, there is a “hierarchy” of trades and the painters are near the bottom, therefor the butt of most jokes.