It's all Greek to me.

ジュリアス・シーザーを見に行くことができたのですが、演劇そのものの感想はまた別の機会で。シェイクスピア劇のあるあるは、ことわざ表現が登場することです。というか、シェイクスピア劇が初出のケースなのでシェイクスピアが本家なのですが。。。ジュリアスシーザーで登場したのはGreek to meでした。

BBCの動画が丁寧に説明してくれています。

(Wikipedia)

Origins

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 dwindling among those who were copying manuscripts in monastic libraries.[citation needed]

Recorded usage of the metaphor in English traces back to the early modern period. It appears in 1599 in Shakespeare's play Julius Caesar, as spoken by Servilius Casca to Cassius after a festival in which Caesar was offered a crown:

CASSIUS: Did Cicero say any thing?

CASCA: Ay, he spoke Greek.

CASSIUS: To what effect?

CASCA: Nay, an I tell you that, I'll 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.

ここでもう一つ紹介されていたのが、次の表現です。

(ウィズダム）

〖通例the ～; 単数扱い〗 (古代ローマ暦で)(3, 5, 7, 10月の)15日, (ほかの月の)13日.

Beware the Ides of March.

3月15日を警戒せよ〘Julius Caesarが暗殺前に受けていた警告(Shakespeare作, Julius Caesarより); 凶事の警告〙.

（ロングマン）

Ides of March, the

March 15th, famous for being the day on which Julius Caesar was killed by a group of his former friends because they thought he had too much power. Caesar is supposed to have been warned by a fortune-teller to ‘beware the Ides of March’. People sometimes use this expression when giving a warning.

It's all greek to meに関しては今でも使えるイディオムですが、面白いのは各言語によって難しい言語というのは変わること。Wikipediaは言語ごとの類似表現を紹介してくれていました。中でも中国語が標的になることが多いようです。

（Wikipedia）

In other languages

In an article published by Arnold L. Rosenberg in the language journal Lingvisticæ Investigationes, he claimed that there was a popular "consensus" that Chinese was the "hardest" language, since various non-English languages most frequently used the Chinese language in their equivalent expression to the English idiom "it's all Greek to me".[1] Also, David Moser of the University of Michigan Center for Chinese Studies made the same claim as Arnold L. Rosenberg.

日本語として紹介されていたのは「ちんぷんかんぷん」。諸説あるようですが、難関な漢語を用いる説がここではとられています。

Japanese

ちんぷんかんぷん

chin pun kan pun [tɕimpɯŋkampɯɴ]

"Ching chong"

Formal speech (referring to the "Chinese" sound of incomprehensible Chinese loanwords used by the scholarly élite)

Yutaが個人的に驚きだったのは、Greek to meといった演説がキケロによるものだったこと。シェイクスピアの訳本では「シセロ」となっていますが、日本だと「キケロ」ですよね。そうカエサルと同時代の人でした。