I could be wrong, but I don't think the word existed before this song. As far as the meaning of it for the song, I would say it means the same as yippee or hooray. She is breaking up with her boyfriend. A guy who's a real hooligan.
In the song she sings "Mais la je m'intoxe...........et lui que son maque", which if I'm correct means if I poisoned myself............he would just mock me, which shows you what a moron he is and why she's so happy to be rid of him.
I found another translation of the song where it says this line stands for that she is intoxicated and he makes fun of her but whichever one is right, it's conveying the same message, she's in a bad state and he's making fun of her.
You really have to look at the French version of the song to get the better meaning of what the word youpidou is trying to convey because it's one of the songs that when Mylene Farmer rewrote the French version into the English version, the song didn't make a whole lot of sense. It certainly didn't convey the same story the French version did.
Maybe one of our French speaking members like Bigdan can help out with a better translation of this song.
Here's a translation I have on file, but it's far from perfect. Problem being people interpret things different ways.
Take a look at the line J'ai le moral au grand zero, which they translate a I am feeling really down which is close but a more literal translation is my moral is a big zero. I usually try and find a translation of a song, then I'll look up the words individually in a French to English dictionary and then I can pretty much figure out for myself exactly what message the song is trying to convey.
I myself would like to have a better understanding of what the line g'noux cailloux means.