Actually, in the classical period, armor didn't have pants. Ever.
Instead, what warriors would wear is a breastplate to cover their chest, then around their pelvis/groin was the tasset, usually made from leather straps.
Greek Hoplites
Sometimes it was just the lower tunic with leather or metal plates hanging down.
Imperial Roman Legionary
The shins would be protected by strapped on armor plates called greaves.
Even in Late Medieval/Early Renaissance armor the thighs would be protected in the front with a metal tasset covering the hips and upper thighs. The groin would be covered with a chainmail skirt. Sometimes a codpiece might be used.
The problem with the idea of armored pants is the human upper thighs are a ball and socket joint that calls for much better mobility than could be provided by protective metal plates, especially for mounted warriors. There's no such thing as armored pants for this reason, just a variety of different approaches to provide some protection while keeping the wearer free to move about unrestricted. Better to avoid getting hit than have to absorb damage!
So whenever I've seen the phrase "gird up thy loins" I took it to mean "put on your tasset" as part of getting suited up for battle.
(I don't mean to wreck the joke... it was a good one. I just thought people might be interested in the reason behind it.)
