What you think you are saying:
"Ayyyyy! I'm the fuckin' Fonz!"
What you are actually saying:
"Ayyyyy! I'm going to jam my thumb in your anus!"
What the hell?
It's not just the Middle East. This seemingly universal gesture is also hideously offensive in West Africa and South America, whose citizens would doubtless get really confused if they ever watched Ebert and Roeper. "This movie is great, Bill! So great that I'd like to anally rape it with my thumb!"
The thumbs-up sign has been confusing people for thousands of years. Contrary to Hollywood legend, Roman gladiators were not spared by a thumbs-up, but by a hidden thumb. If the origins of both gestures are linked, we can only assume this meant, "Do not kill the prisoner, he seems the perfect solution to the emperor's arthritic finger."