“Double down” comes from blackjack, where you double your bet on a strong hand...

https://www.golf-bookmarks.win/double-down-comes-straight-from-blackjack-you-double-your-bet-after-seeing

“Double down” comes from blackjack, where you double your bet on a strong hand to boost winnings. Language quirk: learners often think it means playing it safe, when it actually means taking a bigger risk. Use it when you’re all in, not cautious.