Technically, "rex" in Latin means "king," not "lord." "Lord" in Latin would be "Dominus." "Mundi" gives more the sense of "World" than of "Earth"--though interchangable in English, "earth" would be "terra"--meaning "land," "dirt," and "planet." The correct translation of this phrase is "King of the World." Which of course lends credability to my theory that Leonardo DiCaprio is a demon from hell, sent to torture serious moviegoers.