“Double down” comes straight from blackjack—when a player doubles their bet...

https://atomic-wiki.win/index.php/Above_Board_Meaning_and_Origin:_Gambling%E2%80%99s_Influence_on_Business_English

“Double down” comes straight from blackjack—when a player doubles their bet after seeing the first two cards, hoping for a strong hand. Learners often misuse it to mean “just try harder” generally. Nope