There are quite a few sites that discuss this article of clothing. "Breech cloth" was used
most often, but "Breech clout is probably the older term". It was commonly of wool, and
there was nothing between the thighs. I'll bet the original wearers would have been glad
to have had modern jockey shorts, if only for exclusion of biting flies.
Calling it a "butt flap" sounds like adolescent contempt.