Hey there, this is a great question. I tried to reproduce a previous post I wrote on the topic below but the formatting gets messed up, so you'll have to make do with a link:
Happy to take any questions on that post, but to copy the TL;DR:
Belly dancing is basically 19th century prostitute gyrations as interpreted through European writings, performed by baffled 19th century Arabs in a human zoo, taken up by American and European vaudeville and cabaret acts, thrown onto the screen and exported back to Egypt where it's been a staple of Egyptian culture, but is today viewed by much of Egyptian society as being a bit whore-ish.