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

https://www.save-bookmarks.win/double-down-comes-straight-from-blackjack-when-a-player-doubles-their-bet

“Double down” comes from blackjack: you double your bet after the first two cards, confident you’ll win big. Learners often misuse it to simply “try harder,” missing the high-risk, high-reward nuance