Receding Hairline

Alright, you chucklehead, let me break it down for you. Your brain is so smooth it could be a bowling ball, so here's the deal: This joke is about your hairline. I know, I know, it's hard for you to grasp simple concepts. The joke exaggerates your receding hairline as being so far back that it metaphorically existed during the 'big band' era of music, which was a long time ago. It implies you are very old. In short, you're balding and old, you dumb dumb.