What does "it's all Greek to me" mean?

[Tina]

Test No. incompl/elem-31 "Responses (4)", question 7

Mike: 'I assume you're following the argument, aren't you.'
Jane: '.........'

(a) No, it's all German to me.
(b) No, it's all Greek to me.
(c) No, it's all Spanish to me.
(d) No, it's all Latin to me.

Test No. incompl/elem-31 "Responses (4)", answer 7

Mike: 'I assume you're following the argument, aren't you.'
Jane: 'No, it's all Greek to me.'

Correct answer: (b) No, it's all Greek to me.

Your answer was: incorrect
Mike: 'I assume you're following the argument, aren't you.'
Jane: 'No, it's all German to me.'
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Hi Mr. Alan

What does "it's all Greek to me" mean?

Thank you
Tina

[Murle]

It means.....Being such that understanding or comprehension is difficult or impossible; incomprehensible..

[Snoopy_Blah_Blah]

Heh, I've chosen German. I understand it's an idiom here.
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