As Jim mentioned, it's a reference to craps.
The point is that rolling a seven (two dice together having seven eyes up) can be both a winning or a losing throw in this game, depending on the timing. It is also the most common throw.
So rolling them sevens doesn't mean winning or losing, it means playing or gambling.
Gambling with nothing to lose has some connotations: it either means you cannot lose the bet - or it means you have nothing left that you can lose. Normally you cannot enter a gambling game if you cannot put up the stakes, so if you are gambling while you have nothing to lose, you are completely broke and risking it all. Not very careful or weary :)
So, you rolled them sevens with nothing to lose means as much as you went on playing even though you couldn't / shouldn't have.
As for why you would normally stop if you have nothing left to lose, as medica asked, the last thing that you have (otherwise actively playing is difficult) might be your life. Since serious gamblers don't like to win the game and win nothing for it (which happens if the loser has nothing to pay them with), they may well consider it fair to relieve you
from that life. So, while it may
seem that gambling with nothing to lose is risk-free, it may be a lot riskier than gambling when you have money!