It means, it can't exist in the US.
<span>Mainly because the Constitution is the supreme law the US and anything that goes against any amendment in the Constitution is unconstitutional and therefore cannot exist as proposed.
From Dementiax on Yahoo Answers</span>
Ooo, history! I got this one :D
The stamp act was what required a tax to be paid if you exchanged documents. The act was made by the congress, and it allowed those who paid their tax to have a stamp on their document!