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

https://noon-wiki.win/index.php/Play_Your_Cards_Close_to_Your_Chest:_The_Origin_and_Use_of_This_Classic_Concealment_Idiom

"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