“Sorry?" barked Hagrid, turning to stare at the Dursleys, who shrank back into the shadows. "It's them as should be sorry! I knew yeh weren't gettin' yer letters but I never thought yeh wouldn't even know abou' Hogwarts, fer cryin' out loud! Did yeh never wonder where yer parents learned it all?"
This is from Harry Potter. I think "fer cryin' out loud" is "for crying out loud", but I don't know how to relate it to the context. How should we understand it in the context?
"For crying out loud" indicates the speaker is surprised or irritated. Cambridge Dictionary's definition is pretty clear:
I am annoyed or surprised by this:
Oh, for crying out loud, would you stop leaving your magazines all over the house!
Similar idioms include: for Christ's sake, for God's sake, for Pete's sake, and so on and so forth.
So that person in your quoted text is saying:
How did you not know about Hogwarts? How have you not heard about Hogwarts? This is where your parents received their training. Why hasn't anybody told you?
