“Double down” comes from blackjack, where you double your bet after seeing your...

https://wiki-book.win/index.php/059_Gambling_Vocabulary_That_Became_Mainstream_English

“Double down” comes from blackjack, where you double your bet after seeing your first two cards, betting bigger on a strong hand. Many learners misuse it to mean ‘try twice’—it’s really about committing more when you’re confident, not repeating an action.