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

https://yenkee-wiki.win/index.php/Wild_Card_Meaning_in_Sports_vs._Everyday_Speech

“Double down” comes from blackjack: you double your bet after seeing your first two cards, betting on a strong hand. Learners often misuse it to mean 'try harder' broadly, but it’s really about risking more when the odds look good