Greek

A time that is expected to never arrive or occur. (Used after a preposition, especially "at," "on," or "till.") A reference to the day of the new moon and the first day of the month in the ancient Roman calendar, which the Greeks did not observe. You lent that leech John more money? It'll be at the Greek calends when he pays you back.

Prov. Do not trust an opponent who offers to do something nice for you. (A line from the story of the Trojan horse, as told in Vergil's Aeneid.) Jill: I can't believe Melanie brought me cookies today, when we've been fighting for weeks. Jane: Beware of Greeks bearing gifts. She probably has ulterior motives. When the rival company invited all his employees to a Christmas party, Tom's first impulse was to beware of Greeks bearing gifts, but then he upbraided himself for being paranoid.

incomprehensible to someone; as mysterious as Greek writing. I don't understand this. It's all Greek to me. She said it was Greek to her, and that it made no sense at all.

something that you say when you do not understand something that is written or said I've tried reading the manual but it's all Greek to me.

Also, it's all Greek to me. It is beyond my comprehension, as in This new computer program is all Greek to me. This expression was coined by Shakespeare, who used it literally in Julius Caesar (1:2), where Casca says of a speech by Seneca, deliberately given in Greek so that some would not understand it, "For mine own part, it was Greek to me." It soon was transferred to anything unintelligible.

Unintelligible, as in “I didn't understand a word he said—it was all Greek to me.” Shakespeare said it best in this exchange from Julius Caesar: Cassius: Did Cicero say any thing? Casca: Aye, 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.