Mary,

If you're asked about "the great Tetrarch Ali G", you can at least quote the bafflement of the Mudcat as proof that the meaning of the phrase is evidently lost to the modern world. The amount of random knowledge combined with serious scholarship here is awesome, and if there were any serious explanations they would almost certainly crop up here.

For what it's worth, tetrarch is a Greek term, whereas Ali is obviously an Arab name, so I'm sceptical about that explanation for the mystery phrase. But I take it that you're interested in all other aspects of the background to the song, so I defer to the knowledgeable in this respect.