For me a real loincloth is a long strip of material passing through the legs and held up by a cord or string tied around the waist. For example Johnny Weissmuller wore a real loincloth, but it look like Gordon Scott wore flaps over briefs or speedo.
Thumbs up to Bob T, but there are the self-holding, tied ones like a bahag or a rokushaku fundoshi, but as Tarzan fashion, they're out of question, of course :)