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

https://www.toro-bookmarks.win/double-down-means-to-bet-more-when-the-stakes-get-tough-it-comes-from

“Double down” comes from blackjack, where you double your bet after seeing your first cards, hoping to beat the dealer. It means taking a bigger risk to win big. Learners often say “double up” instead—a close miss, but it means something else entirely.