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

https://foxtrot-wiki.win/index.php/Gambling_Terms_That_Became_Business_Jargon:_From_Card_Tables_to_Corporate_Boardrooms

“Double down” comes from blackjack, where you double your bet after seeing your first two cards, aiming for a bigger win. It means you’re committing fully, often against the odds