“Double down” comes straight from blackjack: you double your original bet after...

https://idiominsider.com/talking-odds-at-work-the-risk-idioms-every-english-learner-gets-slightly-wrong/

“Double down” comes straight from blackjack: you double your original bet after seeing your first two cards, hoping for a strong hand. Language learners often misuse it to mean just “try harder,” but it’s about risk—doubling stakes, not effort.