The origin of this idiom is a bit unclear, but it likely stems from the fact that ancient Greek was a very complex and challenging language for those who didn't speak it. For many people, Greek would have been as good as a foreign language, hence the idiom's meaning.
Here are some examples of how "Greek to me" can be used:
* "My boss's instructions were Greek to me. I have no idea what he wants me to do."
* "The legal documents were filled with jargon that was Greek to me."
* "This new software is Greek to me. I can't even figure out how to open it."
It's worth noting that the idiom is not meant to be offensive or disrespectful to Greek people or the Greek language. It's simply a humorous way to express confusion and bewilderment.