Christos is the Greek word for "Anointed," which we will recognize from the familiar word "Christ." This perfect passive participle comes from the infinitive chriein, which means "to anoint."
In the Old Testament, kings and priests were anointed with oil to mark their special calling. The kings of Israel were thus anointed of God, and the ultimate King to whom all Jews looked for deliverance (the King of Kings) came to be known as the Anointed one.
In Hebrew the word is Mashiach, which became Christos when the Hebrew O.T. was translated into Greek in the 3rd century BC. Of course, Mashiach became Messiah in English.
So "Christ" is not Jesus' last name; it is his rightful title. It might make things easier to call our Lord "Jesus the Anointed" -- which would do justice to the biblical context of the word, and remove the obfuscation occasioned by the merely transliterated terms Messiah and Christ. (I suspect that whether this idea will ever catch on or not, is doubtful.)
