That terrorist with the dent in his head?

I think everyone knows who I'm talking about. That Egyption surgion turned terrorist who has a giant dent in his head, bounty :25mil. I think he's named Al-Zaquakawi or something similar to that. Anyway does anyone know the story behind that hideous dent in his head? I haven't been able to find an explanation online, and me and my friend were trying to figure it out, like did someone hit him in the face with an ugly stick or something?