Actually it comes from the Latin title “Caesar”.
So does “Kaiser”, by the way.
You’re absolutely right. But it means emperor. Historically, the Kaiser had more than one kingdom, each ruled by a king.
This doesn't violate the Constitution because the person granting the titles is not a foreign ruler. And the title "czar" might be considered a title of ignobility rather than one of nobility.
Latin, etc.
I have AGAIN begun watching the HBO DVD Series “Rome” - first and second seasons. Disturbing parallels in the history of both “Republics.”