Meaning of 'for crying out loud'

[Guest]

Test No. incompl/inter-43 "A Crying Shame", question 6

For crying out ......... how many times do I have to tell you how to work that machine?

(a) strong
(b) loud
(c) full
(d) hard

Test No. incompl/inter-43 "A Crying Shame", answer 6

For crying out loud how many times do I have to tell you how to work that machine?

Correct answer: (b) loud
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please explain the sentence construction.

[Torsten]

For crying out loud is an idiom which expresses impatience or annoyance.
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