Another (more) common English idiom for this is _It's all Greek to me_ and Eijiro offers an interesting word which I haven't encountered before:
> Don't ask me anything about computers. It's **all Greek to me**.
> ****
The theory seems to be that the word appeared in the Edo era and made fun of Confucianists who liked to use a lot of obscure words of Chinese origin, or possibly mimicked the unintelligible speech of foreigners.
However, this word seems to be mostly about speech, or the ideas being explained, and not the writing itself.