> butt flap

LOL! I thought this referred to the opening in one of those really old fashioned, full body undergarments which can be unbuttoned when one needs to visit the outhouse/loo/WC.
Loincloth, breechcloth, and even breechclout seem to all refer to the same kind of modest covering for one's private area. As far as I know these cover virtually nothing round the backside - thus 'back/butt flap' or the like is pretty ironic.
A modern term for the loincloth might be a g-string of course, although these seem to be only very narrow version thereof. Not that I have any personal experience in the field!