Well, I have read through the entire thread and I learned it in 7th grade in Chorus class in North Carolina, US. It was 1972 and this song was in our music textbook/songbook. As it was explained to us it was a song that the white men used in encouraging each other to capture more Africans/Zulus so that they could be shipped overseas to become slaves. I was always very uncomfortable singing this one, as it seemed terribly inappropriate to sing at all, and even worse to be taught to adolescents in a public school in the US in 1972. Slavery was abolished in 1865. And to top it off, the teacher was Black. And there we sat- singing "Hold him down, the Swazi warrior, hold him down, the Swazi King!"
And the time period: our schools were only just in the process of integrating. There was a good deal of violence associated with that in our school the following year.
I was thinking about this song and Googled and stumbled upon this thread. I don't believe I read that any other poster here was from the US. I was really surprised that no one else knew it to be a song associated with slavery.
I hope to get some feedback on this post.