Slow Mum

Your mum is so slow, it took her nine months to make a joke.

Explanation

Explain Bear

Okay, so basically, the joke is saying your mom is slow. Like, REALLY slow. It usually takes nine months to make a baby, but the joke implies it takes that long for your mom to even come up with a simple joke. And based on how you laughed at this joke, you're probably as slow as she is.