Any of several ancient styles of garment designed to cover the genitals. Usually made from a folded and tied length of cloth, sometimes held in place by a belt. If it has a dangling length of cloth hanging in front and/or or behind, that part is called, for Danbooru's purposes, a pelvic curtain.
Fundoshi are a range of specifically Japanese styles of loincloth. The loincloth's modern replacements are panties, briefs, and boxers.
Both loincloth and fundoshi nowadays are used as festival clothing (usually with happi, or alone) , as a swimsuit (when worn at the beach) and occasionally, as underwear (usually worm under kimono, or Japanese armor etc). Make sure to use the appropriate tag when tagging them.
If the loincloth is attached to a full-body clothing garment, consider tagging tabard as well (post #7734925).
