What does "proper" mean in the Necessary and Proper Clause?

The Necessary and Proper Clause can be found in Article I, Section 8 of the Constitution. Sometimes called the Elastic Clause, it states that Congress has the power "to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof" (Article I, Section 8, Clause 18). This clause makes flexible, or stretches (makes "elastic") the power of Congress to make laws regarding situations not explicitly stated in the Constitution.
The true meaning of this clause was questioned in the Supreme Court case McCulloch v. Maryland. In this 1819 case, the court ruled in favor of McCulloch, agreeing that the establishment of a national bank by Congress was "necessary and proper" and that the state of Maryland did not have the right to tax the bank.
In this case, Chief Justice John Marshall delivered the opinion of the court, clarifying the meaning of “necessary and proper.” Marshall stated,

Let the end be legitimate, let it be within the scope of the Constitution, and all means which are appropriate, which are plainly adapted to that end, which are not prohibited, but consist with the letter and spirit of the Constitution. (Opinion, McCulloch v. Maryland)

Based on this, we can conclude that the “proper” portion of the clause is interpreted to mean “appropriate.”
https://www.law.cornell.edu/supremecourt/text/17/316