gird (up) one's loins, to

Also, gird up one's loins. Prepare oneself for action, as in I'm girding up my loins for that crucial interview. This expression comes from the Bible (Proverbs 31:17) and originally alluded to tucking up the traditional long robe into a girdle (that is, a belt) so it will not hamper physical activity. [c. 1600]

To summon up one's inner resources in preparation for action.

To prepare for action (hard work, a journey, warfare). The term comes from the Bible, both Old and New Testaments, and uses gird in the sense of “encircle with a belt or band.” The ancient Jews wore loose clothing and put on a girdle, or belt, only when they went to work or set out to travel. Thus, “He girded up his loins, and ran” appears in I Kings (18:76), and “Gird up now thy loins like a man” in Job (in several passages). It had already become figurative in the New Testament, where 1 Peter has it, “Gird up the loins of your mind, be sober” (1:13).