(Isa. 21:5)
If the tunic was ungirded it would interfere with a person’s ability to walk freely, and so a girdle was always worn when leaving home for any kind of a journey (See 2 Kgs. 4:29; Acts 12:8). There were and are today two kinds of girdles. One, a common variety, is of leather, usually six inches broad and furnished with clasps. This was the kind of girdle worn by Elijah (2 Kgs. 1:8), and by John the Baptist (Matt. 3:4). The other, a more valuable variety, is of linen (See Jer. 13:1), or sometimes of silk or embroidered material. It is generally a handbreadth wide. The girdle served as a pouch in which to keep money (2 Sam. 18:11) and other things that might be needed (Mark 6:8). The girdle was used to fasten a man’s sword to his body (1 Sam. 25:13)… .
The Scriptures often make symbolic use of the girdle. When Jesus said to His disciples: “Let your loins be girded about” (Luke 12:35), it was as if He had said: “Be as men who have a long race to run; gather up the folds of your flowing robes, and fasten them with your girdle; that nothing may keep you back or impede your steps.” In Bible language, “to be girded” means: “to be ready for action” (cf. Ps. 18:39) … Paul calls truth to be the Christian’s girdle in his warfare with Satan (Eph. 6:14).
(Fred H. Wight, Manners and Customs of Bible Lands [Chicago: Moody Press, 1953], 93–94.)