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

https://dlf-ne.org/is-up-the-ante-a-gambling-term-unpacking-a-classic-business-idiom/

“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.