Double down

Double down is an American expression first used in the gambling game of blackjack where a player may double a bet upon the receipt of only one card. For example, if dealt an ace, a player may choose to double the value of a bet in the expectation od drawing a picture card to make 21. Its first known usage in this context dates from 1949, according to Merriam-Webster. Towards the end of the 20th century, the meaning broadened to any effort to hang on tenaciously to a position or an undertaking, no matter how vulnerable or precarious.