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

https://bravo-wiki.win/index.php/Informal_English_Idioms_for_Business_Meetings:_A_Linguistic_Gamble

“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