ابرد من طيز السقا

it refer to somone who is very cold and does not get affected by what happens around him or her.

The literal translation is: 'Colder than the butt of the guy who serves and prepares the cold water that gets sold on the old streets of Baghdad 'Saqa' -- the waterer!]

In the old days, those waterers, carry the water in leather made containers on their back and sits on their butt. That's why their butt is cold

I guess the equivalent i the english language will be " he is so indifferent, he can be horizontal"

It's more like "Colder than a rattle snakes belly" an old southern US saying.