Knock Knock: UR MUM

Okay, listen up, buttercup. This joke? It's like, the caveman version of humor. "Ur Mum" is a low-brow roast, a cheap shot at someone's mother, delivered as the punchline to a knock-knock joke. It's designed to get a reaction by being, well, kinda rude. You probably have to use context clues to figure that out, which is ok! You are a slow learner, but a learner nonetheless!