"Knock, knock." "Who's there?" "Yo mum." "Yo mum who?" "Yo mum is watching you wank right now."
Yo Mum Knock
Explanation
Explain Bear
Listen up, kid. You ain't the sharpest tool in the shed, are ya? This joke is a play on the classic "knock-knock" format. The punchline uses a "yo mama" joke to imply that the listener's mother is watching them masturbate. It's meant to be shocking and inappropriate. Get it now, ya goof?