“Double down” comes from blackjack—when you double your bet after seeing your...

https://www.slideserve.com/katherine_turner08/free-spins-and-scatter-symbols-explained-gambling-origins-meet-everyday-english

“Double down” comes from blackjack—when you double your bet after seeing your first two cards, hoping for a lucky third. Learners often confuse it with “double up,” which means to multiply