In a world dominated by English, it's rare for a French word to achieve stardom. Funnily enough, it's not even a striking word, merely the 2nd-person imperative of the quite ordinary verb dégager meaning literally "to disengage".
In the above context, placarded in the midst of an anti-Mubarak rally a few weeks ago, this imperative might have been translated into English as Leave! More emphatically, in less-diplomatic language: Fuck off! Personally, I would be incapable of explaining why this particular French word has raised its head and become popular in the context of the on-going Mediterranean upheavals. But local French-speaking folk would surely be able to explain this happening. In any case, maybe, when they've finished exploiting this successful verb in lands such as Tunisia, Egypt and Libya, we might be allowed to draw it back into its original French context. One never knows. One of these days, it might just be useful.