Orphan: What are you doing tonight?
Me: Your mum... oh wait, you don't have one.
Orphan's Mother
Explanation
Explain Bear
Alright, listen up, you absolute donut. The joke is that the dude was gonna say he's doing the orphan's mom, but then he remembers the orphan doesn't HAVE a mom. It's funny because it's mean, and you probably think that's peak comedy, you sad sack of potatoes.