The word translated in the KJV as "loins" refers to the lower back part of the abdomen, above the waist. There has been a widespread misconception that it refers to reproductive organs.
Some (such as J. Bergman Kline) consider the blood and water which poured out of Christ's ruptured sides to be a picture of how Christ begat us (James 1:18), since blood and water are associated with physical birth. They relate the Old Testament
usage of the word "loins" for begetting to the rupture of Christ's side.