What is Ad Jura Regis?

Legal Definition
To the rights of the king; a writ which was brought by the king's clerk, presented to a llving, against those who endeavored to eject him, to the prejudice of the king's title. Reg. Writs, 61.
-- Black's Law Dictionary
Legal Definition
A writ by one holding a living from the king directed against those seeking to deprive him of it.
-- Ballentine's Law Dictionary