Yo mama's so fat, she wrestled a polar bear and won.
Yo Mama vs Polar Bear
Explanation
Explain Bear
Listen up, buttercup. This "yo mama" joke is all about exaggerating how fat someone's mom is. Like, so fat she can beat up a whole polar bear. It's funny because it's ridiculous, get it? Polar bears are strong! And you not so much... hehe.