Greek to me

That's Greek to me or It's (all) Greek to me is an idiom in English, expressing that something is not understandable.

The idiom is typically used with respect to the foreign nature, 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 (either ancient or modern), 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, unlike for example the dormant metaphor "foot of the hill", which permits saying "that hill has a tree at its foot".