It is commonly used in reference to a complex or imprecise verbal or written expression, that may use unfamiliar jargon, dialect, or symbols.
While most scribes were familiar with Latin, few people in medieval Western Europe, even among the intellectual classes, were schooled in Greek.
When copying classic manuscripts they would frequently encounter passages and quotations in Greek which they would have no way of translating, and as such would note the phrase in the margins.
There was more foolery yet, if I could remember it.Here, Casca's literal ignorance of Greek is the source of the phrase, using its common meaning to play on the uncertainty among the conspirators about Cicero's attitude to Caesar's increasingly regal behaviour.
This is an example of the usage of demonyms in relation to the ability of a people to be understood, comparable to the development of the words barbarian (one who babbles), Nemec (Slavic for "the mute one," indicating Germans).