Gird up the loins, brace yourself for vigorous action, or energetic endurance. The Jews wore loose garments, which they girded about their loins when they travelled or worked.
“Gird up the loins of your mind.”- 1 Peter i. 13.
My little finger shall be thicker than my father's loins
(1 Kings xii. 10). My lightest tax shall be heavier than the most oppressive tax of my predecessor. The arrogant answer of Rehoboam to the deputation which waited on him to entreat an alleviation of “the yoke” laid on them by Solomon. The reply caused the revolt of all the tribes, except those of Judah and Benjamin.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894