Jambo (Swahili greeting),

Retarded = relatively slow in mental or emotional or physical development (synonyms: unintelligent, stupid).  A perfect fit for this joke I would say.

Here is another bonus retarded joke:

Joke teller: Do you want to hear a dirty joke?  (Maybe you do. Maybe you don’t.  It doesn’t matter.  There is no way to halt the telling of this joke.)  X number of white horses fell in the mud.

To joke teller: Did they? Well, that’s not a dirty joke. That’s dirty horses. Let me know when you can tell the difference and I’ll consider speaking to you again.

That’s all I’ve got for now.  I hope you enjoyed my Retarded Jokes series.  Its done.  Coming soon, a new series that I call Safety Nut Thrill Seekers.