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

https://www.fastbookmarks.win/double-down-comes-from-blackjack-you-double-your-bet-when-you-re-confident

“Double down” comes from blackjack: you double your bet after seeing your first two cards, risking more for a bigger win. Learners often say “double up” instead, which means something else entirely. Use “double down” to show commitment, not just increase.