Gird up the Loins
(To). To prepare for hard work or a journey. The Jews wore a girdle only when at work or on a journey. Even to the present day, Eastern people, who wear loose dresses, gird them about the loins.
“The loose tunic was an inconvenient walking dress; therefore, when persons went from home, they tied a girdle round it (2 Kings iv. 2; ix. 1; Isaiah v. 27; Jeremiah i. 17; John xxi. 7; Acts xii. 8).” —Jahn: Archeologia Biblica (section 121).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894