ANSWER: Unfortunately, the New Testament cannot give us a direct meaning of the abbreviation INRI. This is because these letters represent Latin, not Greek, words. The book of Matthew, however, does gives us an idea of what was placed over Jesus' head when they hung him on the cross. Pontius Pilate, the Roman authority in Judea, commanded that the following accusation against Christ be written above his head, "This is Jesus, the King of the Jews" (Matthew 27:37, HBFV throughout).
The gospels of Mark (Mark 15:26) and Luke (Luke 23:38) have similar wording compared to Matthew. While the references in Matthew and Mark do not reveal to us which languages the accusation was written in, Luke and John DO tell us. In John's "version" of INRI he states that, hung above Jesus' head, was the phrase "Jesus the Nazarean, the King of the Jews" (John 19:19). This phrase was written in the languages of Hebrew, Greek and Latin (verse 20).
As an interesting side note, the phrase "King of the Jews" can be found eighteen times in the four gospel accounts. The book of John accounts for one third of its New Testament uses. The phrase is first used in the Bible by the Persian Magi when they were seeking Christ in order to worship him. Unsure exactly where he is located, they travel to Jerusalem and ask Herod the Great "Where is the One Who has been born King of the Jews?" (Matthew 2:2).
One reason why the abbreviation INRI is used to represent the charge against Christ is that scholars generally used the wording presented in the Gospel of John as opposed to Matthew, Luke or Mark. They did this because they felt the reference to Christ's hometown of Nazareth, in the accusation against him, were a more complete and accurate recording of the charge against him than the other three gospel accounts.
Another reason why INRI, extracted from the book of John, is used is due to an early translation of the Scriptures into the Latin language. Jerome, in the late fourth century A.D., translated the original language text of the Bible to Latin, creating the Vulgate (Latin) translation. It became such an important translation, especially for the Catholic Church, that in the 16th century they made it their official Bible.
So, how does the Latin abbreviation INRI represent the official charge against Jesus, used by the Roman government, in order to justify condemning him to receive the death penalty?
The letter "I" represents Jesus' name spelled Iesus. Strangely, even though Latin has a letter "J" in their alphabet, it was decided to spell Christ's name beginning with an "I." This is probably because Greek and Hebrew do not have a letter "J" in their alphabets. The letter "N" stands for Nazarenvs, which in English is Nazareth or Nazarene. The letter "R" stands for Rex, which translated is "king." The last letter 'I' is an abbreviation for the word Ivdaeorum, which translated is "Jews" (again, using "I" in place of using of "J"). In English, the literal meaning or definition of INRI is "Jesus Nazareth King Jews."