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

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

“Double down” comes from blackjack, where you double your bet after seeing your first two cards, risking more for a better payoff. Learners often misuse it to mean just “try harder,” but it’s about backing your move with serious stakes, not vague effort.