prepare and strengthen yourself for what is to come
This expression is of biblical origin – the idea being that the long, loose garments worn in the ancient Orient had to be hitched up to avoid impeding a person's movement.
In 1 Kings 18 : 45 - 6, we find : And Ahab rode and went to Jezreel. And Elijah girded up his loins and ran before Ahab to the entrance of Jezreel. The phrase was also used metaphorically in the New Testament.
Wherefore gird up the loins of your mind - be sober and hope to the end for the grace that is to be brought unto you. (Peter 1 : 13)