I know you know better, Barbarian, I was just “ragging” on you. 
Hey congratulations on your new baby! I just looked at it from another point of view: a belt with rags may not count as an “undergarment,” exactly, but it is “something.”
Just following up on the lack of bathing, a lot of medievel saints made a big deal out of the mortification of the flesh and a rejection of the physical world. If you look at the cliche picture of a decadent Roman or early medieval nobleman, you would think about the Roman baths, perufmes, banquets, and elaborate and colorful clothing. So the saints made a point of not bathing, and had no problem wearing filthy clothes. Add to that the fact that the typical soap was actually rather caustic, and clothing with linings or padding, to be effectively washed, had to be unsewn, washed, and resewn. OK if you are, say a king or a rich merchant, kind of tuff if you are some poor peasant and that is the only coat you own.
Bathing, I believe, also ran into problems with the medical profession because it interfered with the concepts of humors and balance - you know, things were supposed to be warm and dry, or cold and dry, whatever. Stepping into a bath tup screwed that up, so - don’t bath unless you have to.
Several saints of the middle ages made a big deal out of not bathing, one Italian saint claimed she never took a bath in her life. She also claimed she went months without a bowel movement, which must have impressed a different sort of following. My grandfather said that it wasn’t until WWI that he took a bath or shower more than once a week - courtesy of the US Army.
Bathing in the middle ages was a social and pleasurable activity, and I think, recommended for health. It’s true that there were people who didn’t bathe as a form of corporal mortification, but it was uncommon. Here’s an essay about bathing before 1600: