"Loins girt about with truth" or "belt of truth buckled around your waist"
Ephesians 6:14 in the KJV says:
"Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness;" (Ephesians 6:14, KJV)
Modern translations tend to avoid the word "loins" and opt instead for "waist". This is done presumably in order to modernize the language. This is unfortunate because in doing so they obscure a rich biblical metaphor of the "loins". The following literal translations mention the "loins":
YLT: "Stand, therefore, having your loins girt about in truth...."
LITV: "Then stand firm, "having girded your loins about with truth"...."
The "loins" (Hebrew: חָלָץ chalats; Greek: ὀσφύς osphus) were a big deal for the ancient Hebrews and Greeks. The following definition of the Greek word for "loins" in HELPS Word-studies is helpful:
"osphýs – properly, the hip (reproductive area); used figuratively in 1 Pet 1:13 of the "reproductive" (creative) capacity of the renewed mind (cf. Ro 12:1-3)."
"(osphýs) is "the seat of generative power (Heb 7:5,10, Abbott-Smith). "To smite the loins" referred to a fatal blow – "forever ending" anything that would (could) come from the slain."
The "loins" are properly the area of the lower back but could refer to a larger area including the groin. The "loins" were therefore regarded as the source of reproductive power. The following Bible passages refer to this reproductive function of the "loins":
Genesis 35:11: "And God said unto him, I am God Almighty: be fruitful and multiply; a nation and a company of nations shall be of thee, and kings shall come out of thy loins;"
1 Kings 8:19 (also 2 Chronicles 6:9): "Nevertheless thou shalt not build the house; but thy son that shall come forth out of thy loins, he shall build the house unto my name.
Acts 2:30: "Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne;"
Hebrews 7:10: "For he was yet in the loins of his father, when Melchisedec met him."
Sometimes, the "loins" were simply regarded as the strong core of the human body without necessarily referring to its reproductive function:
Proverbs 31:17: "She girdeth her loins with strength, and strengtheneth her arms."
Now, with respect to the "belt of truth", the metaphor of the full armour of God is not a new concept in Ephesians 6. Isaiah used the imagery first, as in the following passages:
Isaiah 59:17: "For he put on righteousness as a breastplate, and an helmet of salvation upon his head; and he put on the garments of vengeance for clothing, and was clad with zeal as a cloak."
Isaiah 11:5: "And righteousness shall be the girdle of his loins, and faithfulness the girdle of his reins."
In Isaiah, it is the promised Messiah or God who is equipped with these armour pieces to reveal his glory. Also, Isaiah probably envisions an Oriental (Hebrew) armour style. In Ephesians, it is the Christian who is equipped with these armour pieces for spiritual protection. And the imagery is probably based on the Roman armour style (rather than Hebrew) because Paul was writing to an audience under Roman rule. So the Ephesian armour imagery is probably based on the Isaiah imagery but with some new twists applied by Paul. Paul likely applied Roman armour functionality to his imagery of the armour of God.
The Roman armour included a belt with a protective portion hanging over the groin. The belt served several functions, such as tying the undergarments and upper armour pieces together, holding the sword, and protecting the groin area. Because of these multiple functions of the Roman belt, Bible interpreters have diverse interpretations of the belt of truth. Some interpreters say the belt of truth symbolizes the fact that we need truth to keep everything else in our lives together. Some interpreters say the belt of truth symbolizes the fact that we need truth in order to wield the sword of the spirit. Some interpreters say the belt of truth symbolizes the fact that we need truth to guard our "loins". It may well be that all of these symbolisms are intended. Despite these various other interpretations, a straightforward interpretation of the belt of truth is that it protects the "loins" because it literally mentions "loins".
The idea of "girding up your loins" comes from the Old Testament. It means to be prepared for action. The imagery is that of a person with a long flowing robe tying up the skirt around the waist so that the legs can move freely:
Job 40:7 "Gird up thy loins now like a man: I will demand of thee, and declare thou unto me."
The phrase "gird up the loins" appears in the New Testament at 1 Peter 1:13:
1 Peter 1:13-15: "Wherefore gird up the loins of your mind, be sober, and hope to the end for the grace that is to be brought unto you at the revelation of Jesus Christ; As obedient children, not fashioning yourselves according to the former lusts in your ignorance: But as he which hath called you is holy, so be ye holy in all manner of conversation;"
Here, most interpreters agree that the intended meaning of "gird up the loins of your mind" is to prepare the mind for action. But there may be a deeper meaning. The rest of 1 Peter 1:13-15 closely ties the act of "girding up the loins of your mind" to the act of being sober and refraining from lusts (or passions, or desires) and becoming "holy in all manner of conversation". There is also evidence that ancient Greeks and Romans had a custom of binding up the "loins" to refrain from sexual activity in order to preserve the body's energy for athletic competitions: Secord, Jared "The Celibate Athlete: Athletic Metaphors, Medical Thought, and Sexual Abstinence in the Second and Third Centuries CE"
Given the close association of the "loins" with reproduction, the full meaning of "girding up the loins of your mind" could mean to prepare the mind for action by binding up one's distracting desires. Tying up your skirt around your waist will help you move your legs freely, but it also serves a double purpose of making it impossible to carry out any sexual activity. This is not to say that sexual activity is to be refrained completely in life, but that there is a time and place. A soldier going out to war shouldn't be dilly-dallying with distracting sexual thoughts.
That brings us back to the belt of truth. Given how important the "loins" were to the ancient Hebrews and Greeks for their reproductive function, and given how the Roman belt served the function of protecting the groin, and given how 1 Peter 1:13-15 relates the act of girding the loins to containing one's lusts, and given how the Bible strongly links the departure from truth to sexual immorality (Romans 1:21-25), it is a valid and valuable interpretation to think that the belt of truth is needed to protect our reproductive energy from falsehood. That means we need the belt of truth to dispel the lies Satan tells us about sexuality.
There is even a ministry called "Belt of Truth Ministries" that aims to help men struggling with pornography addiction:
Belt of Truth Ministries
https://www.beltoftruth.com/
We need to wear the truth of God over our "loins" because our sexual drive could be the most irrational part of us and most susceptible to false ideas. Practically, this means speaking:
truth about your identity,
truth about how to really overcome your shame and insecurities,
truth about relationships,
truth about God's purpose for your body,
truth about what truly satisfies your soul,
truth about what really leads to happiness.
While this interpretation that the "belt of truth" protects one's "loins" aka sexuality is indeed an interpretation, it is a valid interpretation that should not be obscured by the translation. If one wishes to interpret the "belt of truth" any other way, the use of "loins" would still permit it. The best translation is one which preserves what is there in the text.