Gird Up His Loins

Tonight, while sitting on the couch, Anna (Nate's 9 year old sister) came upstairs and whispers to me "We (Anna and Libby, Nate's 13 year old sister) are reading our scriptures downstairs, what does 'gird up his loins' mean?" I just laughed so she asked Nate and he answered that it meant that he was getting dressed. Anna then looked relieved and said "Oh, Libby was making it sound horrifying."