“Double down” comes from blackjack, where you double your bet after seeing your...

https://blogfreely.net/margaret-bennett/h1-b-gambling-terms-that-became-business-jargon-from-card-tables-to

“Double down” comes from blackjack, where you double your bet after seeing your first two cards, betting on a stronger hand. People often misuse it to mean “try twice as hard” generally, but it’s really about risk—doubling what’s already on the table