Why are g strings underwear called g strings?

Basically, the etymology is unknown. The Word has been around since the 1800s. It was used to refer to the string holding American Indian loincloths in place and later was used to refer to the loincloth itself. Other speculation includes that it may have been an analogy to the thickest string on a violin or a way of saying the taboo Words "groin" or "girdle."