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

It may have been a direct translation of a similar phrase in Latin: "Graecum est; non legitur" ("it is Greek, [therefore] it cannot be read"). This phrase was increasingly used by monk scribes in the Middle Ages, as knowledge of the Greek alphabet and language was decreasing among those who were copying manuscripts in monastic libraries.

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.