Like any legal document, the words of the Constitution have the same meaning today as they were understood to mean the day it was ratified. While some of those words mean the same today as they did back then, some of them are totally different than what you’d expect.
For example, in Article I, Section 8, Clause 7 of the Constitution, Congress is delegated the power to “establish Post Offices and post Roads.”
Some people make the logical conclusion that a “post Road,” in support of “Post Offices,” is any road over which the mail might travel. And because of that, they believe the federal government has either some or even final say over virtually every road in the country.
But, 18th century “post Roads” were actually quite different.
They were major thoroughfares built for speedy travel between distinct localities. They were different from lesser roads because they had stations, which were called “stages” or “posts” that were used for hiring and changing horses. A modern version of this would be the interstate highway system.
To the founding generation, city streets, cross roads and other lesser highways – even if a courier might carry mail over them – were not “post Roads.”
There are many other examples like this. But, the rule of thumb is this. If you use a dictionary to look up a word or phrase in the Constitution, make sure it was published around the time of ratification, or you’ll likely end up thinking the feds have far more power than they’re supposed to.