In human anatomy the words loin and loins have come to refer to that part of the human body below the rib cage. Originally it may have been more clearly defined as the area between the bottom of the ribs and the pelvis. The term also has become euphemistic for human genitals due to their prominence in that anatomical region. So a simple article of clothing that is worn around the genital area has been named a loincloth.
In the Authorized King James Version of the Christian Bible the term "loins" is used frequently. It is suggested that the "loins" is the minimum one must cover on a person's body to be respectful of the law. According to the Goddess Wiki this suggests that if a man (or, possibly, a woman as well) covers up just their genital area, they are considered clothed enough for public life. Please allow me to be sexist and insert a slight whoopee at that stage!
I'm not sure she'd really be allowed out in public!
How girded should loins be?
Frequently the expression "fruit of his loins" is used to refer to children.
The term "gird one's loins" was used in the Roman Era meaning to pull up and tie one's lower garments between one's legs to increase one's mobility in battle. In the modern age, it has become an idiom meaning to prepare oneself for the worst.
I quote in full from the excellent Historyzine - the history podcast
"Girding your loins is something you would find people doing a couple of thousand years ago in Near East Asia and around the Mediterranean just before they attempted anything strenuous and physical such as going into battle. The floor length robes would quite likely get in the way so they would need to be tucked out of the way.
This was accomplished by pulling up the fabric of the knee-length tunic so the length in front stopped at your upper thigh, and collecting the excess material in your front. (You pull the material forward so the back of your tunic is snug against your backside.) Next, you tuck the extra front material down between your legs and gather it behind you. At this point, you collect half of the material behind you evenly on each side of your back (left and right). The final step involves wrapping each side of the material around your waist and tying it together in front.
This gets you ready for battle and the robes are neatly tucked out of the way.
It makes sense from this that the phrase has now come to mean getting yourself prepared for some trial to come.
So there you go.
Now if ever you need to gird your loins you’ll know how it’s done."