But, for my own part, it was Greek to me
It was unintelligible to me.
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
Each country has a notional language that is symbolic of being especially incomprehensible. In England we have double Dutch. In fact Dutch is one of the easier languages to learn for English speakers and this position is bequeathed more for political and social than linguistic ones. Shakespeare is making use of the fact that the characters are Roman and, as far as the audience would expect, couldn't (or wouldn't want to) speak Greek. The phrase 'it's all Greek to me' is now commonplace.
Other authors used a similar phrase around the same time, so it can't be said that Shakespeare coined it. There is also a mediaeval Latin phrase which translates as 'It is Greek; it cannot be read', which is earlier and could be said to be the origin of the term.
See other phrases and sayings from Shakespeare.