From a Christmas Carol (pages 24-25 of the Google edition). The Spirit of Christmas Past awakens Scrooge who is temporarily confused about what day it is, then reassured:

This was a great relief, because "Three days after sight of this First of Exchange pay to Mr. Ebenezer Scrooge or his order," and so forth, would have become a mere United States' security if there were no days to count by.

Meaning an utterly worthless piece of paper.

Wonks Anonymous believes that the US gained this reputation during the 1830's. After he was elected Andrew Jackson acted to destroy The Second Bank of The United States in 1833. Instead states were allowed to charter their own banks and regulate credit and money markets as they sought fit while the funds of the Treasury were deposited in various carefully selected state banks. We would call it deregulation.

As with most financial innovations, this one created a boom with widespread borrowing, mainly concentrated in the construction of canals. It was followed by The Panic of 1837 during which time United States Securities acquired the reputation alluded to in Dickens.