Albert Barnes 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)

Gird up thy loins — This is a symbol of preparation for earnest exertion, also implying firm purpose and a degree of alacrity.

Be not dismayed ... — This means literally, “Be not dismayed at their faces, lest I dismay thee before their faces.” Naturally despondent and self-distrustful, there was still no feebleness in Jeremiah’s character. There was in him a moral superiority of the will, which made him, at any cost to himself, faithfully discharge whatever his conscience told him was his duty.