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

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

"Double down" comes from blackjack, where you double your bet after seeing your first two cards, aiming for a big win. Learners often misuse it to mean 'do something twice'—not quite. It’s about commitment, not repetition