וְאַתָּה֙ תֶּאְזֹ֣ר מָתְנֶ֔יךָ וְקַמְתָּ֙ וְדִבַּרְתָּ֣ אֲלֵיהֶ֔ם אֵ֛ת כָּל־אֲשֶׁ֥ר אָנֹכִ֖י אֲצַוֶּ֑ךָּ אַל־תֵּחַת֙ מִפְּנֵיהֶ֔ם פֶּֽן־אֲחִתְּךָ֖ לִפְנֵיהֶֽם׃
그러므로 너는 네 허리를 동이고 일어나 내가 네게 명한 바를 다 그들에게 고하라 그들을 인하여 두려워 말라 두렵건대 내가 너로 그들 앞에서 두려움을 당하게 할까 하노라
Tze'enah Ure'enah
“So you, gird up your loins, etc.” [1:17]. You Jeremiah, strongly gird up your loins, stand up and speak to them everything that I have commanded you. You should have no fear before them. That is to say, do not be afraid and you will believe that I can protect you. However, if you will be afraid, then I will make you fearful.
Rashi
And you shall gird your loins This is an expression of quickening like a man of valor.