A high risk / high reward operation. The phrase originates from craps
where a hard six is achieved by rolling threes on a pair of six-sided dice (3+3). Rolling a hard six has a probability of about 3% whereas rolling six by any other combination has about a 14% chance. A hard six pays 7 to 1 whereas a regular six pays only 7 to 6.
The phrase was used by the character Commander/Admiral William Adama in the television series Battlestar Galactica