It's Greek to me!

Origin

The notion of something being utterly incomprehensible by likening it to a foreign tongue dates back centuries, even before English existed as we know it. Ancient Romans, for instance, sometimes referred to Greek texts as unreadable. Yet, it was the masterful pen of William Shakespeare that etched 'It's Greek to me!' into the English lexicon forever. In his play Julius Caesar, when Casca is pressed to explain what Cicero said, he dismisses it with the famous line, 'But, for mine own part, it was Greek to me.' This memorable retort perfectly encapsulated the feeling of complete bewilderment, ensuring that Greek would forever symbolize the ultimate linguistic barrier for English speakers, even if they'd never studied a word of it.