Wikipedia
In C. S. Lewis's Chronicles of Narnia, a Tisroc is a ruler of Calormen. His position is most like that of a Pharaoh, in that he is an absolute monarch, and is believed to be descended from the Calormene god Tash. Whenever a Calormene subject speaks of the Tisroc, he adds "may he live for ever," and it is considered blasphemy not to say this. For example, in The Horse and His Boy, Aravis says, "Now this Ahoshta is of base birth, though in these later years he has won the favor of the Tisroc (may he live for ever) by flattery and evil counsels..." Enemies of the Tisroc take pleasure in not saying this, however, especially while in his domain. As the talking horse Bree put it: "... why should I talk slaves' and fools' talk? I don't want him to live forever, and I know he's not going to live for ever whether I want him to or not."
Lewis made up a name for the ruler and nobility of Calormen while still referring to the Tisroc's son Rabadash as a prince. Since elsewhere the sons of Tisrocs are styled Tarkaan (lord), the title "Prince" would appear to be reserved for the heir apparent. A female counterpart for the Tisroc is never mentioned. It is implied that the Tisroc has many wives.
Other titles for nobility of Calormen are Tarkaan and Tarkheena. A Tarkaan is supposed to be related to the Tisroc and seems to have authority over provinces in Calormen. Tarkheenas are either wives or daughters of Tarkaans.
The practice of saying "may he live forever" after the Tisroc's name is borrowed from Edith Nesbit's description of Babylonian customs in her time-travel story The Story of the Amulet. It possibly stems from the Aramaic formula "King, live forever" (malkâ, lʻâlmîn heyî) found in the Biblical book of Daniel.
The word Tisroc may owe something to Nisroch, the name given in the Bible to a deity of Assyria, who also appears as a demon in Paradise Lost.