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

https://page-wiki.win/index.php/Why_Do_People_Misspell_%22Up_the_Ante%22%3F

"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