This is a great cliché meaning to steel oneself for the worst.  People have seen fit to cover their loins for, well, just about ever.  (gird (up) your loins)  And with good reason.  But how tightly one covers her or mostly his loins is a question of fashion, and perhaps the weather.  But if you are going to represent with your homies, it’s probably not going to be good if your pants fall down right when you’re getting ready to throw down.  Therefore, you had best tighten your belt, or gird (which means to tighten) your loins (as in loin cloth).  This is also the origin of “girdle.”  (gird)

Work Cited

“gird.”  2014.  Vocabulary.com.  01 Sept. 2014.  http://www.vocabulary.com/dictionary/gird

“gird (up) your loins.”  2014.  The Free Dictionary.  01 Sept. 2014.  http://idioms.thefreedictionary.com/gird+loins