"Double down" comes from blackjack, where players double their bet to back a...

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

"Double down" comes from blackjack, where players double their bet to back a strong hand. In English, it means to commit more effort or resources when things get risky. Common learner slip? Using it as “double up,” which changes the meaning completely