Meaning: It was unintelligible to me.

Origin: From Shakespeare`s Julius Caesar, 1601:

CASSIUS

Did Cicero say any thing?

CASCA

Ay, he spoke Greek.

CASSIUS

To what effect?

CASCA

Nay, an I tell you that, Ill ne'er look you i' the face again: but those that understood him
smiled at one another and shook their heads; but, for mine own part, it was Greek to me.
I could tell you more news too: Marullus and Flavius, for pulling scarfs off
Caesar's images, are put to silence. Fare you well. There was more foolery yet, if I could remember it.