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

https://scarlettsultimateperspectives.huicopper.com/how-casino-culture-shaped-the-english-language

"Double down" comes from blackjack, where you double your bet after seeing your first two cards, betting on a strong hand. Learners often misuse it to mean just "try harder," but it’s about risk—doubling stakes, not effort