“Double down” comes straight from blackjack: you double your bet when feeling...

https://wiki-site.win/index.php/Risk_Idioms_in_English_Explained:_From_the_Poker_Table_to_the_Boardroom

“Double down” comes straight from blackjack: you double your bet when feeling confident, risking more for a bigger win. Learners often confuse it with ‘double up,’ thinking it means ‘to increase’ anywhere. Nope