Everyone's heard the supposed fact that if you take the English idiom "It's Greek to me" and search for equivalent idioms in all the world's languages to arrive at a consensus as to which language is the hardest, the results of such a linguistic survey is that Chinese easily wins as the canonical incomprehensible language. (For example, the French have the expression "C'est du chinois", "It's Chinese", i.e., "It's incomprehensible". Other languages have similar sayings.) So then the question arises: What do the Chinese themselves consider to be an impossibly hard language? You then look for the corresponding phrase in Chinese, and you find Gēn tiānshū yíyàng 跟天书一样 meaning "It's like heavenly script."
David Moser, Why Chinese is So Damn Hard