Is it true the definition of the word must actually means may according to the us supreme court?

If the law says "You must stop at a red light," it both grants you permission to stop at the red light and requires you to stop at the red light. You do not have an option of not stopping.

That's where the tax protester nonsense about "must" means "may" comes from. Must always means you are required to do the act. It also gives you permission to do the required act.