What does Luke 1:43 mean?

Elizabeth has been in joyful seclusion for several months, awaiting the birth of her only child (Luke 1:24–25). This unborn son will be known as John the Baptist (Luke 3:2–3). Elizabeth has just been visited by her relative, Mary, an unmarried virgin (Luke 1:26–27). Mary is also pregnant, through the miraculous work of the Holy Spirit, with the very Son of God (Luke 1:31–33). When Mary announces herself, Elizabeth's unborn son moves dramatically, "leaping" in the womb. Elizabeth also responds to the presence of the unborn Jesus Christ, shouting out a blessing on Mary (Luke 1:39–42).

Here, this unborn Child in Mary's womb is referred to as Elizabeth's "Lord." The literal Greek term kyrios can refer to a master or be used in place of the English word "sir." In context, Elizabeth is celebrating Jesus as her Messiah and Savior (Luke 2:11; John 20:28). She realizes that the Messiah her own son, John, will proclaim, is growing inside of Mary's body at that very moment.