John Gill Commentary Jeremiah 1:17

"Thou therefore gird up thy loins, and arise, and speak unto them all that I command thee: be not dismayed at them, lest I dismay thee before them." — Jeremiah 1:17 (ASV)

You therefore gird up your loins. The loins both of his mind and body. The allusion is to the custom of the eastern countries in wearing long garments, who, when they went about business, girt them about them for quicker dispatch; and here it designs haste and expedition in doing the Lord's work, as well as courage and resolution of mind.

And arise; and go from Anathoth to Jerusalem: and speak unto them all that I command you; (See Gill on Jeremiah 1:7), be not dismayed at their faces; (See Gill on Jeremiah 1:8),

Lest I confound you before them; show resentment at him in some way or another, which would make him ashamed before them. The Septuagint and Arabic versions add, "for I am with you to deliver you, says the Lord", as in (Jeremiah 1:8) .