“Double down” comes from blackjack: you double your bet after the first two...

https://www.4shared.com/office/12SjSzBTge/pdf-11845-28864.html

“Double down” comes from blackjack: you double your bet after the first two cards, confident you’ll win big. Learners often misuse it to simply “try harder,” missing the high-risk, high-reward nuance