Matthew Poole and continuators on Exodus 13:18

18 But God led the people about through the way of the wilderness of the red Sea, and the children of Israel went up harnessed out of the land of Egypt.

Or, armed, or girt with swords and belts about the fifth rib, as the Hebrew word may imply. But it doth not appear how or whence they should get their arms, nor how the Egyptians would permit so numerous a people to have and to keep arms, especially when they had a long time oppressed and exasperated them, and made them desperate. It is true, some few of them might procure arms, but this word is here used concerning the whole body of them. Others render it, by fives, five and five in a rank, that is, by an usual Synecdoche, in military order, not doubtfully and fearfully but confidently and couragiously, not confusedly, as men that steal or run away, but in good order, so as one might not hinder another. Which interpretation is strengthened by comparing Ios. 1. 14. It may be rendred, girt, to wit, about the fifth rib, as the word implies, the place where men used to gird their garments, this being the usual posture for travellers: He implies that they went out resolved upon and prepared for their journey.

Marginal references: chap. 14. 2.; Or, by five in a rank.