I just read another explanation for "girding your waist with truth," and rather than writing a whole new post I'm just going to post this as an addition. I think they can both be true.
Because girding your waist meant pulling up and tying off the tunic so that the worker or soldier would not stumble, girding with truth can also mean to be spiritually sharp and alert so that we won't stumble.
"Therefore girding up the loins of your mind, being sober, perfectly hope for the grace being brought to you at the revelation of Jesus Christ, as obedient children, not fashioning yourselves according to the former lusts in your ignorance, but according to the Holy One who has called you, you also become holy in all conduct, because it is written, 'Be holy, for I am holy.'" (1 Peter 1:13-16)
I would welcome any other thoughts on this.