Actually, "to gird one's loins" originally meant to put on the accoutrement of war, the sword belt which hung low across the loins. I can't discover exactly how old it is, but I would suggest Anglo-Saxon or Norman.

Older I think, CapK - the imagery is used in the Bible as well, at Ephesians 6:14, and 1 Peter 1:13, according to a interlinear translation I have.