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

https://wiki-aero.win/index.php/Hedge_Your_Bets:_Finance_Idiom_vs._Everyday_Use

"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