Bongo-bongo land is a fictional African country popular in both racist slurs and parodies of the same. The phrase was popularised by the late Alan Clark PC during an impassioned speech while serving as a Conservative Member of Parliament in the British government. This is most likely to have taken place in his tenure as member for Plymouth Sutton, but the exact date eludes me for the moment. The coining of the phrase seems to be lost in the mists of time, but wertperch informs me that it was in common use in the 1960's at least.
The word is intended in a humourous way, but depending on the speaker is also horrifyingly racist or ruthless mocking of xenophobic politics. The original phrase used was "send them back to Bongo-bongo land" and this is the most common use of the word in use today.
This is not, by any stretch of the imagination, a thing you want to say in the presence of an impressionable youth, potential employer or somebody you're courting. Well, maybe the last one would be ok if she's got a cracking sense of humour or is a bit of a toff.
The power of the term for mocking of racism is in using it in ridiculously inappropriate situations. For example, the last time I used it was at breakfast earlier this year. Sitting next to a half-british friend who was brought up in Venezuela. The details of the situation are lost in the mists of my memory, but there was a moment of social confusion that prompted somebody to mutter
Just an affectionate ribbing from when we lived with a few foreign students
. She once replied "You wish you were fucking foreigners". Still don't know if that was a grammatical error, but that's neither here nor there. To which I added
"Yeah, go back to bongo-bongo land"
To re-emphasise, don't use this unless your audience will get the reference (I wouldn't like to have to explain it to a group of confused foreigners, would you?) and only if the joke will be appreciated. Used sparingly, incorrectly and in good company it's a wonderful comic device. Otherwise it's disciplinary proceedings at work, suspension from school, etc. If you do screw up and get in trouble for it though, let me know, chances are I'll find it hilarious!