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

https://remote-wiki.win/index.php/Why_an_Idiom_Acquired_from_a_List_Sounds_Unnatural

“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.