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

https://troysbrilliantjournal.trexgame.net/up-the-ante-or-up-the-anti-which-is-correct

“Double down” comes from blackjack: you double your bet after the first two cards, betting on a strong hand. Learners often say “double up,” which isn’t right—double down means commit more risk, not just multiply something. Use it when you’re all in.