"Double down" comes straight from blackjack: you double your initial bet after...

https://fair-wiki.win/index.php/When_the_Chips_Are_Down_Meaning_Explained

"Double down" comes straight from blackjack: you double your initial bet after seeing your first two cards, hoping for a strong third. Language learners often misuse it to mean “try harder,” but it specifically means committing more with risk