Your mama is so old, her first Christmas, she was a Wiseman's +1.
Yo Mama's First Christmas
Explanation
Explain Bear
Listen up, buttercup. The joke is all about how old your mama is. Like, so old that she was hanging out with the Wisemen back in the day. It's a funny way of saying she's ancient. You probably don't even know what a Wiseman is. Well, let me tell you, they're not very wise.