What does the idiom 'It's (all) Greek to me' express?

'That's Greek to me' or 'it's (all) Greek to me' is an idiom in English, expressing that something is difficult to understand.

The idiom is typically used with respect to something of a foreign nature, however it may be considered an insult to some. Complexity or imprecision of verbal or written expression or diagram, often containing excessive use of jargon, dialect, mathematics, science, symbols, or diagrams. The metaphor makes reference to Greek, as an archetypal foreign form of communication both written and spoken. Technically, the phrase is classified as a dead metaphor, meaning that its components cannot be used separately.

The contribution of Greek to the English vocabulary can be quantified in two ways, type and token frequencies: type frequency is the proportion of distinct words; token frequency is the proportion of words in actual texts.

Since most words of Greek origin are specialized technical and scientific coinages, the type frequency is considerably higher than the token frequency. In a typical English dictionary of 80,000 words, which corresponds very roughly to the vocabulary of an educated English speaker, about 5% of the words are borrowed from Greek.

Of the 500 most common words in English, 18 are of Greek origin: place, problem, school, system, program, idea, story, base, center, period, history, type, music, political, policy, paper, phone, economic.

You may wish to consider: Is it all Greek to you or do you speak Greek without knowing it?