Adam Clarke on Jeremiah 1:17

Gird up thy loins - Take courage and be ready, lest I confound thee; take courage and be resolute, פן pen, lest by their opposition thou be terrified and confounded. God is often represented as doing or causing to be done, what he only permits or suffers to be done. Or, do not fear them, I will not suffer thee to be confounded. So Dahler, Ne crains pas que je te confonde a leurs yeux, "Do not fear that I shall confound thee before them." It is well known that the phrase, gird up thy reins, is a metaphor taken from the long robes of the Asiatics; which, on going a journey, or performing their ordinary work, they were obliged to truss up under their girdles, that the motions of the body might not be impeded.