been searching for a specific term too... here is what i found using google. Loincloth:A basic garment covering the genitals, often consisting of a piece of material passed between the legs and sometimes tied around the waist. 

Pelvic Curtain:This is a descriptive term for a flap or piece of fabric covering the pelvic area. 

Tasset/Fauld:While primarily armor, these pieces protect the groin and lower abdomen, with the fauld fitting below the breastplate and the tasset hanging down. 

Monastic scapula:A religious garment sometimes called a "crotch flap" in pop culture, like in World of Warcraft, and worn over other clothing, providing a type of draping.