The lower abdomen, including the reproductive organs. A man's loins are described as the source of his offspring <Gen. 35:11>, (KJV). When a man was ready to work, he put a belt around his waist; then he tucked up his robe so his legs would not be hindered. This is the meaning of "girded up his loins" <1 Kin. 18:46>. To "gird up the loins of your mind" <1 Pet. 1:13> means to prepare for strenuous mental activity.
(from Nelson's Illustrated Bible Dictionary)
(Copyright (C) 1986, Thomas Nelson Publishers)