This has got to be one of the strangest metaphors in the whole Bible. I have heard it said of some men that they think only with their sexual organs, but I somehow doubt that is what Peter was driving at.
Below is how a number of Bible scholars weigh in on this figure of speech:
Walls says, “Gird up the loins of your mind is a metaphor suited to men of the Middle East, who wore long gowns which were 'girded up' when energetic effort was contemplated. The equivalent in our world is obviously to 'roll up one's sleeves' or to 'take off one's coat to it'. We must begin to act as those who mean business. Note that it is in the realm of the mind that this serious new activity is demanded. Conversion to Christ and regeneration by the Spirit are meant to be accomplished by mental awakening, by a new exercise of powers of understanding now divinely released and renewed (cf. Rom xii.2; Eph. iv. 17, 18, 23).”
“One who 'girds the loins' is one who hems up the long garment which hinders one's stride; thus, dianoia, one's thinking and willing, should be 'rolled up' in preparation for departure. ...The closest parallel in content to I Peter 1:13 is Lk. 12:35: 'Let your loins be girded and your lamps burning' in expectation of the Lord. So too Eph. 6:14, though without a direct eschatological focus.” (Goppelt)
M.E. Boring feels that this phrase is 'reflecting the stance of the Israelites as they ate the first Passover '[with] their loins girded,' that is, ready to travel...The initial command is thus something like 'roll up your (mental) sleeves,' to prepare for disciplined intellectual work. The beginning of their historical pilgrimage is a matter of hard thinking, but not something they are to figure out for themselves; it is a response to the word of God that comes through preaching, that is, through the church's communication of the gospel.”
“According to an ancient tradition, the children of Israel were to celebrate the Passover in this fashion [with their loins girded] to show that they were ready for imminent departure (Exod xii 11). This tradition may have influenced First Peter. It is even possible that this figure of speech reflects the author's awareness of the custom of arranging baptismal services at Eastertime, the exodus and the passing through the Red Sea being prototypes of Christian baptism (I Cor x 2). The admonition to gird the loins would then rest in part on the assumed baptismal situation. Baptism was also celebrated in the early church by symbolically dressing in a certain costume (as probably reflected in Gal iii 27 where baptism is compared to the putting on of Christ, and in Eph vi 11, 13-17, with its description of the Christian's armor). However, even without the support of such rough associations it is evident from the illustration that the admonition relates to the preparedness of the Christians for the journey that faces them.” (Reicke)
Davids similarly explains, “In Israel an ordinary person wore as the basic garment a long, sleeveless shirt of linen or wool that reached to the knees or ankles. Over this a mantle something like a poncho might be worn, although the mantle was laid aside for work...but for active service, such as work or war, it was tucked up into a belt at the waist to leave the legs free (I Kings 18:46; Jer. 1:17; Luke 17:8; John 21:18; Acts 12:8). Thus Peter's allusion pictures a mind prepared for active work...Peter makes it clear that he is using the image as a metaphor by stating clearly 'mind,' which indicates not the intellectual processes in general, but a mental resolve and preparation.”
Keener adds, “ Its mention of the mind emphasizes cognitive readiness, thus keeping the Lord's coming in mind. Keeping in mind the Lord's coming includes the recognition that all must give account for their works. (1 Peter 4:5), and thus urges a perspective that evaluates the present in light of the eternal significance.”
As to the nature of the events for which Christian are to prepare, “The struggle is mental, and Christians are to get their minds ready for battle.” (Dictionary of Biblical Imagery).
“Obedience is a conscious act of the will. Christians in conflict need a tough-minded holiness that is ready for action.” (Raymer)
Polkinghorne: “Hope is more than a vague, amorphous aspiration. It requires that the loins of the mind be girded..., that is that the loose skirts of the flowing robe must be gathered into a belt for hard work or vigorous action...In using the metaphor, maybe Peter recalled his Lord's words to him recorded in Jn 21:18. Thus, abstaining from the enervating pleasure of the surrounding paganism in which they formerly indulged, they must direct their whole being to eternal matters.”
“In view of such a salvation and such good news, Christians are called to exercise mental diligence and moral discipline: this is to be done by looking joyfully and confidently forward to the grace that the revelation of Jesus Christ will bring.” (Wheaton)