11 You will tell me, perhaps, that, high-born as you are, reared in luxury and used to lie softly, you cannot do without wine and dainties, and would find a stricter rule of life unendurable. If so, I can only say: “Live, then, by your own rule, since God's rule is too hard for you.” Not that the Creator and Lord of all takes pleasure in a rumbling and empty stomach, or in fevered lungs; but that these are indispensable as means to the preservation of chastity. Job was dear to God, perfect and upright before Him; yet hear what he says of the devil: “His strength is in the loins, and his force is in the navel.”
The terms are chosen for decency's sake, but the reproductive organs of the two sexes are meant. Thus, the descendant of David, who, according to the promise is to sit upon his throne, is said to come from his loins. And the seventy-five souls descended from Jacob who entered Egypt are said to come out of his thigh. So, also, when his thigh shrank after the Lord had wrestled with him, he ceased to beget children. The Israelites, again, are told to celebrate the passover with loins girded and mortified. God says to Job: “Gird up your loins as a man.” John wears a leathern girdle. The apostles must gird their loins to carry the lamps of the Gospel. When Ezekiel tells us how Jerusalem is found in the plain of wandering, covered with blood, he uses the words: “Your navel has not been cut.” In his assaults on men, therefore, the devil's strength is in the loins; in his attacks on women his force is in the navel.
Source: Letters (New Advent)