What does that even mean?
Also, the US government doesn't really have anything to do with the USD; the USD is managed solely by the Federal Reserve, and the Federal Reserve is not a branch of the US government (it's a privately held bank; look it up).
"it's not nothing"
It's worse than nothing. The Federal Reserve can and does change the value of the dollar at will (it's called inflation). That's why you can buy a loaf of bread with your 5 bucks now, but in 10 years your 5 bucks probably won't even buy a candy bar.
Contrast that with gold. In 1913 (the year the Federal Reserve was created), you could buy a house with 100 ounces of gold. 100 1913 dollars is worth roughly 2 2013 dollars, but 100 1913 ounces of gold is worth exactly 100 2013 ounces of gold, and you can still buy a house with it (100 ounces of gold is $156,740 USD; no, this won't buy you a house in New York, but it will in most other parts of the country). That's what it means for a currency to be backed by something of value. Value doesn't change.