“Double down” comes straight from blackjack: you double your bet when you’re...

https://high-wiki.win/index.php/Above_Board:_Meaning,_Origin,_and_Why_You_Should_Keep_It_Straight

“Double down” comes straight from blackjack: you double your bet when you’re confident your hand will win. Language learners often misuse it thinking it means "do something twice," but it’s about increasing commitment, not repetition