Cannot find any translation for your spelling, however in French casses with an acute accent over the e means breakages - so casses toi would be breakages you - which doesn't make sense.
Tried Spanish - nothing
Portugese - casses means to annul but toi does not exist so doesn't seem to be this.
I know it's French and I stand corrected on this, but I think it's an expression used in the theatre when calling for a break in someone's performance ??
It's spelled "Casse-toi" and means "p1$$ off" like Buildersmate said. It's not Belgian French though, just French, even if I'm sure they use it too in Belgium.