Concise Bible Dictionary:

The part of man that is used to prefigure the seat of strength. Descendants are represented as coming out of the loins of a man (Gen. 35:11; Heb. 7:10). The loins were girded up for action and activity. God said to Job, “Gird up now thy loins like a man” (Job 38:3). In the Christian’s conflict with wicked spirits in the heavenlies he is exhorted to have his loins girt about with truth (Eph. 6:14).