“Double down” comes straight from blackjack, where you double your bet after...

https://wiki-global.win/index.php/All_Bets_Are_Off_Idiom_Meaning:_When_Uncertainty_Throws_Business_Plans_Out_the_Window

“Double down” comes straight from blackjack, where you double your bet after seeing your first two cards, hoping for a sure win. Learners often misuse it to mean “just keep going,” but it’s really about doubling risk with a plan, not blind persistence.