CASSIUS: Did Cicero say anything?
CASCA: Ay, he spoke Greek.
CASSIUS: To what effect?
CASCA Nay, an I tell you that, I’ll ne’er look you i’ th’ 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.
William Shakespeare (1564-1616) English dramatist and poet
Julius Caesar, Act 1, sc. 2, l. 289ff (1.2.289-295) (1599)
(Source)
Not origin, but likely popularizer of the phrase, "It's Greek to me." Similar phrases had been around since Roman days, and through the Medieval period. Many languages/cultures have similar idioms.
