染博士出身 You Were Trained as a Dyer
八戒道：「孽畜，你原來是個染博士出身。」妖精道：「我怎麼是染博士？」八戒道：「不是染博士，怎麼會使棒槌？」八戒道：「孽畜，你原来是个染博士出身。」妖精道：「我怎么是染博士？」八戒道：「不是染博士，怎么会使棒槌？」
逐句解 Line by line
Bajie said, "You wicked beast, you must have started out as a master dyer."
染博士 is a dyer — 博士 was an honorific for a skilled tradesman as well as a court scholar, so it lands somewhere near calling a man a doctor of dyeing. Bajie has set up a joke and is waiting to be asked.
The fiend said, "How am I a master dyer?"
He asks. Of course he asks. The straight line is delivered with complete seriousness by a creature holding an iron pestle.
Bajie said, "If you were no dyer, how would you know how to work a pounding stick?"
棒槌 is the wooden club used to beat cloth in a dyeing yard or at a laundry stone. The joke is that the fiend's weapon is a workman's tool, which is rather rich from a man fighting with a rake.
Three segments of pure music-hall in the middle of a fight. Bajie sets it up, the fiend feeds him the line, and the punch is that his enemy's terrible pestle is laundry equipment.
It is also a fair fight in miniature. Rake against pestle, farm against dye-yard — two creatures beating each other with implements, and only one of them has noticed how funny that is.