1. In blackjack, to double one's wager after seeing one's initial hand of cards, with only one more card allowed to be drawn afterward. Having already lost a massive sum, he decided to double down an already huge bet to try to win back some of his money.

2. By extension, to significantly increase or strengthen effort, investment, or resolve toward some goal, strategy, or action so as to maximize the potential yield as a result. The president made clear that he intends to double down the government's involvement in the war overseas. With the market booming, many companies are doubling down their development in mobile apps and games.