A phrase made popular in recent years by the brilliant comedian Sacha Baron Cohen, star of the HBO special, Da Ali G Show. The show was originally introduced in the UK, then moved to the US, after becoming popular to the point of being hard to fool anyone anymore. The main difference between the UK version and the US version is that in the UK, it had an audience that you could hear laugh, like most tv sitcoms, whereas the US version has no audience reactions. There was also a movie made called Ali G Indahouse before the move to the US. The movie is actually really lame compared to the reality-based series. Cheap humour and... more cheap humour.
If you listen to the song Keep Their Headz Ringin' by Dr.Dre, you will hear a background singer repeating "B-B-B-B-Boyakasha!", which is probably where Ali G got it from.