Gird Your Loins
“Gird” means to prepare for action or danger. Back in Biblical times, men wore tunics that hung very low, not allowing them freedom of movement for battle. To “gird your loins” was to bundle your tunic up and tie it through your legs and around your waist like a pair of shorts so that you can fight more effectively.
Why I Looked It Up
I found a visual guide from The Art of Manliness on Reddit. I had heard the phrase before, most notably when Stanley Tucci’s character yells it in The Devil Wears Prada when Miranda is entering the building.