What is the opposite of “eureka” (I found it)? In other words, how should I say “I lost it” in ancient Greek? Modern Greek is something like το έχασα.

Ryan Guthrie’s is the most correct word.

Another word: ἀπώλεσα. I have lost (something), or, quite felicitously, I have destroyed. Which is why Apollyon, Revelation’s Angel of the Abyss, is not The Loser, but The Destroyer.

Not that absurd a connection. That Modern Greek word, έχασα? Comes from *χαώνω < *χαόω. To turn something into chaos.