Paint Licker

Paint licker was used a while back, and it was used like "window licker" it was the same insult to say the person is dumb/mentally handy capped because they would lick lead paint off the walls and lead paint can cause brain damage. So, it is to say someone is so dumb the only explanation is they are a "paint licker"
We don't use lead paint anymore so the insalt has pretty much gone away