the loins: the lower back and hips, the body's center of physical strength
This word names the band of the body around the lower back and hips. Because the legs and torso meet there, the Hebrews thought of it as the seat of physical power, the place a person braces from when lifting, running, or fighting. Nearly everything the word does grows out of that one picture.
It is the natural place to fasten things. A belt, a sword, a workman's apron, and the rough cloth of mourning are all tied at the loins, so the word shows up wherever someone girds himself or puts on (or strips off) what he wears at the waist.
From the idea of strength comes the most common figure of speech: to gird up the loins means to gather one's long robe and tuck it into the belt so the legs are free, the way a runner or a worker readies himself. It became a stock image for bracing oneself for hard or urgent action.
The same body region is also where the Hebrews located the gut-level feeling of dread. When terror or grief strikes, the loins are said to shake, buckle, or give way. So the one word can carry both readiness and collapse, depending on whether the loins are being braced or broken.