To expand on Toadstool's point, there's an explanation (which I am led to believe is credible by a colleague who has a basic knowledge of Arabic)
here.
(NB, for the fainter hearts amongst us, or those with small children reading over their shoulder, there is a cartoon at the bottom of the page involving a Da'esh fighter holding a severed head in his hand. The most disturbing thing about it is that the fighter looks like Robbie Coltraine playing Hagrid in the Potter franchise, but there y'go)