“Double down” comes from blackjack: you double your bet after seeing your first...

https://www.slideserve.com/katherine_turner08/free-spins-and-scatter-symbols-explained-gambling-origins-meet-everyday-english

“Double down” comes from blackjack: you double your bet after seeing your first two cards, betting on a strong hand. Learners often misuse it to mean 'try harder' broadly, but it’s really about risking more when the odds look good