The letters “INRI” are initials for the Latin title that Pontius Pilate had written over the head of Jesus Christ on the cross (John 19:19). Latin was the official language of the Roman Empire.
The words were "Iesvs Nazarenvs Rex Ivdaeorvm." Latin uses “I” instead of the English “J”, and “V” instead of “U” (i.e., Jesus Nazarenus Rex Judaeorum). The English translation is "Jesus of Nazareth, the King of the Jews."
The Early Church adopted the first letters of each word of this inscription “INRI” as a symbol. Throughout the centuries INRI has appeared in many paintings of the crucifixion.
Although John’s gospel refers to the writing as a “title,” Mark and Matthew both refer to it as an “accusation.” It was customary to set up over the heads of persons crucified the crime for which they suffered, and the name of the sufferer. The accusation on which Jesus had been condemned by Pilate was his claiming to be the King of the Jews. Ironically, the “crime” for which Jesus was crucified is not a crime at all, but an absolutely true statement. Not only is Jesus King of the Jews, He is the King of all – the King of kings and the Lord of lords (Revelation 17:14 and 19:16). He is King over the entire universe and all its inhabitants. And it was not any crime of His own that was nailed to the cross; it was the crimes (sins) of everyone who would ever put his or her faith in Him for salvation. He has “blotted out the handwriting of ordinances that was against us, which was contrary to us, and has taken it out of the way, nailing it to the cross” (Colossians 2:14)
By the way, Pilate's title for Christ was actually written in three languages(Hebrew, Greek and Latin) .