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

https://www.bookmarkpage.win/double-down-comes-from-blackjack-when-you-double-your-original-bet-after

“Double down” comes from blackjack, where you double your bet after seeing your first two cards, betting on a stronger hand. Learners often mistake it for just trying harder, but it’s really about upping the stakes when odds are in your favor