“Double down” comes straight from blackjack—when you double your original bet...

https://wiki-net.win/index.php/Merriam-Webster_Etymology_of_%22Blue_Chip%22:_Gambling_Origins_of_Everyday_Business_English

“Double down” comes straight from blackjack—when you double your original bet after seeing your first two cards, hoping for a lucky third. Learners often use it to mean just “try harder,” but it’s more about risk and commitment than simple effort