Likely a Hebraism, often used in the King James Bible (e.g., 2 Kings 4:29). Literally referred to the need to strap a belt around one's waist when getting up in order to avoid the cloak falling off.
gird up one’s loins
- (idiomatic, intransitive) To prepare oneself for something demanding.
1857, Herman Melville, chapter 37, in The Confidence Man:
- Man came into this world, not to sit down and muse, not to befog himself with vain subtleties, but to gird up his loins and to work.
1920, Lucy Maud Montgomery, chapter 9, in Rilla of Ingleside:
- Then Susan said briskly, "Well, we must just gird up our loins and pitch in."
- 2004, "Editorial," nepalnews.com (Nepal), vol. 24, no. 6 (Aug 3-19):
- King Gyanendra must gird up his loins and prepare himself for all exigencies.