Double down, with or without the hyphen, has a number of different slang meanings. But the most relevant one appears to be the sense in which it’s used in gambling, in blackjack, to be precise. The literal definition of the gambling term doesn’t make clear, though, at least not to this nongambler, why it should be used metaphorically as it seems to be. And so I found what Urban Dictionary has to say helpful.
One of its definitions for double down is “to engage in risky behavior, especially when one is already in a dangerous situation.” The reference continues, “This figurative usage as appearing in the Oxford English Dictionary is a meaningful explanation of ‘double down’ other than the ubiquitous ‘to double the wager in exchange for only one additional card in Blackjack gambit’ in every online dictionary.”
from The Christian Science Monitor, (2/12/12)
In the news… Sheldon Adelson plans to double down on campaign donations, Salon, 12/5/12