A G-string (alternatively gee-string or gee string) is a type of thong underwear or swimsuit, a narrow piece of cloth, leather, or plastic, that covers or holds the genitals, passes between the buttocks, and is attached to a band around the hips, worn as swimwear or underwear mostly by women, but also by men. The two terms G-string and thong are often used interchangeably; however, they can refer to different pieces of clothing. It is usually worn on beaches, especially in South America and tropical countries.

Read more about G-string:  Etymology

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G-string - Etymology
... The origin of the term "G-string" is obscure ... William Safire in his Ode on a G-String quoted the usage of the word "G-string" for loincloth by Harper's Magazine 15 years after John Hanson Beadle's 1877 usage and ...
Thong (clothing) - Design and Variety - Women's Thongs - G-string
... The G-string style consists of an elastic string, as opposed to a strip of fabric, connecting the front/pouch and the waistband at back ... have been referring to the style of thongs they wore for their performances as G-strings ... The origin of the term G-string is obscure ...