It was the common understanding at the time, in the reference books, etc. You can’t just make up definitions for words in the Constitution that you don’t like. This is akin to the anti-Americans who say “militia” means only the National Guard. The militia was and is every able-bodied free adult male.
Here are some other words not defined in the Constitution:
“the”
“navy”
“ten”
“days”
“two”
“seas”
None of these words are defined in the Constitution. They have and had meanings in their own right.
You don’t get to redefine words in order to twist the Constitution to conform with your anti-American ideals.
But you can, is that it?