“Double down” comes from blackjack—when you double your bet after seeing your...

https://tango-wiki.win/index.php/What_Idioms_Do_Examiners_Dislike%3F_A_Dry_Dive_into_English_Exam_Register

“Double down” comes from blackjack—when you double your bet after seeing your first two cards, betting you’ve got a winner. Learners often misuse it to mean “make more effort” generally, but it’s really about risk, doubling stakes