To gird your loins during the Roman Era meant to draw-up and tie your lower garment between your legs as to increase your mobility and agility. This prevented the loose ends of the tunic from becoming snagged on something or entangled with your feet which would cause you to stumble or fall down when moving about quickly.
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.