The NT writers followed the general usage of the OT, but employed one word, ὀσφύς, to cover the various meanings of the Heb. terms. It indicated the waist (Matt 3:4), the place where garments were tied, but was used metaphorically in Luke 12:35; Ephesians 6:14; 1 Peter 1:13, nevertheless two references (Acts 2:30 KJV; Heb 7:5, 10) mentioned the source of offspring. “Loins” as a literal rendering appears more often in the KJV than in other VSS.
International Standard Bible Encyclopedia (1915)
This variety of Hebrew synonyms seems to be used rather promiscuously for the loins, though there is no little difference in the secondary meanings of these words. They represent various modes of expressing the loins as the seat of strength and vigor (Job 40:16, Hebrew mothen, here used of Behemoth), the center of procreative power, the portion of the body which is girded about, and is considered as specially needful of covering, even under primitive conditions of life (Job 31:20), and where painful disease most effectually unfits a man for work and warfare.
Jacob receives the Divine promise that "kings shall come out of (his) loins" (chalats, Ge 35:11), and we read of 66 souls "that came out of his loins" (yarekh) which went into Egypt (Ge 46:26). The Epistle to the Hebrews speaks of the Levites as having come out of the loins of Abraham (Heb 7:5).
The condition of unfitness for service is described in that the loins (kecel) are filled with a burning (Ps 38:7, the King James Version "loathsome disease"), or that "a sore burden" is laid upon the "loins" (mothen, Ps 66:11). Thus the loins are made "continually to shake" (Ps 69:23), "the joints of (the) loins" (charats) are loosed (Da 5:6), the "loins are filled with anguish" (Isa 21:3). It is very likely that originally a disabling lumbago or the painful affections of the gall or the bladder (calculus, etc.) are meant, but very soon the expression becomes merely metaphorical to express personal helplessness, especially that which can but rely upon assistance and help from God.