A helpful illustrated guide from The Art Of Manliness, what sez:

Back in the days of the ancient Near East, both men and women wore flowing tunics. Around the tunic, they’d wear a belt or girdle. While tunics were comfortable and breezy, the hem of the tunic would often get in the way when a man was fighting or performing hard labor. So when ancient Hebrew men had to battle the Philistines, the men would lift the hem of their tunic up and tuck it into their girdle or tie it in a knot to keep it off the ground. The effect basically created a pair of shorts that provided more freedom of movement. Thus to tell someone to “gird up their loins” was to tell them to get ready for hard work or battle. It was the ancient way of saying “man up!”

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